POLG-related disorders comprise a continuum of overlapping phenotypes that were clinically defined long before their molecular basis was known. Most affected individuals have some, but not all, of the features of a given phenotype; nonetheless, the following nomenclature can assist the clinician in diagnosis and management. Onset of the POLG-related disorders ranges from infancy to late adulthood. Alpers-Huttenlocher syndrome (AHS), one of the most severe phenotypes, is characterized by childhood-onset progressive and ultimately severe encephalopathy with intractable epilepsy and hepatic failure. Childhood myocerebrohepatopathy spectrum (MCHS) presents between the first few months of life and about age three years with developmental delay or dementia, lactic acidosis, and a myopathy with failure to thrive. Other findings can include liver failure, renal tubular acidosis, pancreatitis, cyclic vomiting, and hearing loss. Myoclonic epilepsy myopathy sensory ataxia (MEMSA) now describes the spectrum of disorders with epilepsy, myopathy, and ataxia without ophthalmoplegia. MEMSA now includes the disorders previously described as spinocerebellar ataxia with epilepsy (SCAE). The ataxia neuropathy spectrum (ANS) includes the phenotypes previously referred to as mitochondrial recessive ataxia syndrome (MIRAS) and sensory ataxia neuropathy dysarthria and ophthalmoplegia (SANDO). About 90% of persons in the ANS have ataxia and neuropathy as core features. Approximately two thirds develop seizures and almost one half develop ophthalmoplegia; clinical myopathy is rare. Autosomal recessive progressive external ophthalmoplegia (arPEO) is characterized by progressive weakness of the extraocular eye muscles resulting in ptosis and ophthalmoparesis (or paresis of the extraocular muscles) without associated systemic involvement; however, caution is advised because many individuals with apparently isolated arPEO at the onset develop other manifestations of POLG-related disorders over years or decades. Of note, in the ANS spectrum the neuropathy commonly precedes the onset of PEO by years to decades. Autosomal dominant progressive external ophthalmoplegia (adPEO) typically includes a generalized myopathy and often variable degrees of sensorineural hearing loss, axonal neuropathy, ataxia, depression, parkinsonism, hypogonadism, and cataracts (in what has been called "chronic progressive external ophthalmoplegia plus," or "CPEO+").

D-bifunctional protein deficiency is a disorder of peroxisomal fatty acid beta-oxidation. See also peroxisomal acyl-CoA oxidase deficiency (264470), caused by mutation in the ACOX1 gene (609751) on chromosome 17q25. The clinical manifestations of these 2 deficiencies are similar to those of disorders of peroxisomal assembly, including X-linked adrenoleukodystrophy (ALD; 300100), Zellweger cerebrohepatorenal syndrome (see 214100) and neonatal adrenoleukodystrophy (NALD; see 601539) (Watkins et al., 1995). DBP deficiency has been classified into 3 subtypes depending upon the deficient enzyme activity. Type I is a deficiency of both 2-enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase; type II is a deficiency of hydratase activity alone; and type III is a deficiency of dehydrogenase activity alone. Virtually all patients with types I, II, and III have a severe phenotype characterized by infantile-onset of hypotonia, seizures, and abnormal facial features, and most die before age 2 years. McMillan et al. (2012) proposed a type IV deficiency on the basis of less severe features; these patients have a phenotype reminiscent of Perrault syndrome (PRLTS1; 233400). Pierce et al. (2010) noted that Perrault syndrome and DBP deficiency overlap clinically and suggested that DBP deficiency may be underdiagnosed.

MED12-related disorders include the phenotypes of FG syndrome type 1 (FGS1), Lujan syndrome (LS), X-linked Ohdo syndrome (XLOS), Hardikar syndrome (HS), and nonspecific intellectual disability (NSID). FGS1 and LS share the clinical findings of cognitive impairment, hypotonia, and abnormalities of the corpus callosum. FGS1 is further characterized by absolute or relative macrocephaly, tall forehead, downslanted palpebral fissures, small and simple ears, constipation and/or anal anomalies, broad thumbs and halluces, and characteristic behavior. LS is further characterized by large head, tall thin body habitus, long thin face, prominent nasal bridge, high narrow palate, and short philtrum. Carrier females in families with FGS1 and LS are typically unaffected. XLOS is characterized by intellectual disability, blepharophimosis, and facial coarsening. HS has been described in females with cleft lip and/or cleft palate, biliary and liver anomalies, intestinal malrotation, pigmentary retinopathy, and coarctation of the aorta. Developmental and cognitive concerns have not been reported in females with HS. Pathogenic variants in MED12 have been reported in an increasing number of males and females with NSID, with affected individuals often having clinical features identified in other MED12-related disorders.

Ichthyosis, leukocyte vacuoles, alopecia, and sclerosing cholangitis (ILVASC) is a rare autosomal recessive syndrome characterized by scalp hypotrichosis, scarring alopecia, ichthyosis, and sclerosing cholangitis (summary by Feldmeyer et al., 2006).

Meckel syndrome is an autosomal recessive pre- or perinatal lethal malformation syndrome characterized by renal cystic dysplasia and variably associated features including developmental anomalies of the central nervous system (typically occipital encephalocele), hepatic ductal dysplasia and cysts, and postaxial polydactyly (summary by Smith et al., 2006). For a more complete phenotypic description and information on genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

Classic Joubert syndrome (JS) is characterized by three primary findings: A distinctive cerebellar and brain stem malformation called the molar tooth sign (MTS). Hypotonia. Developmental delays. Often these findings are accompanied by episodic tachypnea or apnea and/or atypical eye movements. In general, the breathing abnormalities improve with age, truncal ataxia develops over time, and acquisition of gross motor milestones is delayed. Cognitive abilities are variable, ranging from severe intellectual disability to normal. Additional findings can include retinal dystrophy, renal disease, ocular colobomas, occipital encephalocele, hepatic fibrosis, polydactyly, oral hamartomas, and endocrine abnormalities. Both intra- and interfamilial variation are seen.

Meckel syndrome is a rare autosomal recessive lethal condition characterized by an occipital meningoencephalocele, enlarged kidneys with multicystic dysplasia and fibrotic changes in the portal area of the liver and with ductal proliferation, and postaxial polydactyly. For a more complete phenotypic description and information on genetic heterogeneity, see MKS1 (249000).

Most people with PFIC3 have signs and symptoms related to liver disease only. Signs and symptoms of PFIC3 usually do not appear until later in infancy or early childhood; rarely, people are diagnosed in early adulthood. Liver failure can occur in childhood or adulthood in people with PFIC3.\n\nThe signs and symptoms of PFIC2 are typically related to liver disease only; however, these signs and symptoms tend to be more severe than those experienced by people with PFIC1. People with PFIC2 often develop liver failure within the first few years of life. Additionally, affected individuals are at increased risk of developing a type of liver cancer called hepatocellular carcinoma.\n\nIn addition to signs and symptoms related to liver disease, people with PFIC1 may have short stature, deafness, diarrhea, inflammation of the pancreas (pancreatitis), and low levels of fat-soluble vitamins (vitamins A, D, E, and K) in the blood. Affected individuals typically develop liver failure before adulthood.\n\nThere are three known types of PFIC: PFIC1, PFIC2, and PFIC3. The types are also sometimes described as shortages of particular proteins needed for normal liver function. Each type has a different genetic cause.\n\nSigns and symptoms of PFIC typically begin in infancy and are related to bile buildup and liver disease. Specifically, affected individuals experience severe itching, yellowing of the skin and whites of the eyes (jaundice), failure to gain weight and grow at the expected rate (failure to thrive), high blood pressure in the vein that supplies blood to the liver (portal hypertension), and an enlarged liver and spleen (hepatosplenomegaly).\n\nProgressive familial intrahepatic cholestasis (PFIC) is a disorder that causes progressive liver disease, which typically leads to liver failure. In people with PFIC, liver cells are less able to secrete a digestive fluid called bile. The buildup of bile in liver cells causes liver disease in affected individuals.

Meckel syndrome is a disorder with severe signs and symptoms that affect many parts of the body. The most common features are enlarged kidneys with numerous fluid-filled cysts; an occipital encephalocele, which is a sac-like protrusion of the brain through an opening at the back of the skull; and the presence of extra fingers and toes (polydactyly). Most affected individuals also have a buildup of scar tissue (fibrosis) in the liver.\n\nOther signs and symptoms of Meckel syndrome vary widely among affected individuals. Numerous abnormalities of the brain and spinal cord (central nervous system) have been reported in people with Meckel syndrome, including a group of birth defects known as neural tube defects. These defects occur when a structure called the neural tube, a layer of cells that ultimately develops into the brain and spinal cord, fails to close completely during the first few weeks of embryonic development. Meckel syndrome can also cause problems with development of the eyes and other facial features, heart, bones, urinary system, and genitalia.\n\nBecause of their serious health problems, most individuals with Meckel syndrome die before or shortly after birth. Most often, affected infants die of respiratory problems or kidney failure.

Meckel syndrome is an autosomal recessive pre- or perinatal lethal disorder characterized by a combination of renal cysts and variably associated features including developmental anomalies of the central nervous system (typically occipital encephalocele), hepatic ductal dysplasia and cysts, and postaxial polydactyly (summary by Baala et al., 2007). For a more complete phenotypic description and information on genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

In general, gallbladder disease (GBD) is one of the major digestive diseases. GBD prevalence is particularly high in some minority populations in the United States, including Native and Mexican Americans. Gallstones composed of cholesterol (cholelithiasis) are the common manifestations of GBD in western countries, including the United States. Most people with gallstones remain asymptomatic through their lifetimes; however, it is estimated that approximately 10 to 50% of individuals eventually develop symptoms. Significant risk factors associated with GBD are age, female sex, obesity (especially central obesity), lipids, diet, parity, type 2 diabetes (125853), medications, and Mexican American ethnicity. GBD appears to be strongly related to the metabolic syndrome (605552) and/or its major components, such as hyperinsulinism, dyslipidemia, and abdominal adiposity (Boland et al., 2002; Tsai et al., 2004). Infection, specifically by Helicobacter, has been implicated in cholelithiasis and cholecystitis (Silva et al., 2003; Maurer et al., 2005). Low phospholipid-associated cholelithiasis is a specific form of gallbladder disease characterized by young-adult onset of chronic cholestasis with intrahepatic sludge and cholesterol cholelithiasis. Affected individuals have recurrence of the disorder after cholecystectomy and show a favorable response to treatment with ursodeoxycholic acid (UDCA) (summary by Pasmant et al., 2012). Mutation in the ABCB4 gene can cause a spectrum of related diseases, including the more severe progressive familial intrahepatic cholestasis-3 (PFIC3; 602347), intrahepatic cholestasis of pregnancy-3 (ICP3; 614972), andoral contraceptive-induced cholestasis (OCIC; see 614972). Genetic Heterogeneity of Gallbladder Disease Two major susceptibility loci for symptomatic gallbladder disease have been identified on chromosome 1p in Mexican Americans (GBD2, 609918; GBD3, 609919). In addition, variations in the ABCG8 gene (605460) on chromosome 2p21 confer susceptibility to gallbladder disease (GBD4; 611465).

This autosomal recessive disorder is designated Meckel syndrome type 7 (MKS7) based on the classic phenotypic triad of (1) cystic renal disease; (2) a central nervous system abnormality, and (3) hepatic abnormalities, as defined by Meckel (1822), Salonen (1984), and Logan et al. (2011). According to these criteria, polydactyly is a variable feature. Herriot et al. (1991) and Al-Gazali et al. (1996) concluded that Dandy-Walker malformation can be the phenotypic manifestation of a central nervous system malformation in MKS. For a general phenotypic description and a discussion of genetic heterogeneity of Meckel syndrome, see MKS1 (249000).

Because of their serious health problems, most individuals with Meckel syndrome die before or shortly after birth. Most often, affected infants die of respiratory problems or kidney failure.\n\nOther signs and symptoms of Meckel syndrome vary widely among affected individuals. Numerous abnormalities of the brain and spinal cord (central nervous system) have been reported in people with Meckel syndrome, including a group of birth defects known as neural tube defects. These defects occur when a structure called the neural tube, a layer of cells that ultimately develops into the brain and spinal cord, fails to close completely during the first few weeks of embryonic development. Meckel syndrome can also cause problems with development of the eyes and other facial features, heart, bones, urinary system, and genitalia.\n\nMeckel syndrome is a disorder with severe signs and symptoms that affect many parts of the body. The most common features are enlarged kidneys with numerous fluid-filled cysts; an occipital encephalocele, which is a sac-like protrusion of the brain through an opening at the back of the skull; and the presence of extra fingers and toes (polydactyly). Most affected individuals also have a buildup of scar tissue (fibrosis) in the liver.

Phosphorylase kinase (PhK) deficiency causing glycogen storage disease type IX (GSD IX) results from deficiency of the enzyme phosphorylase b kinase, which has a major regulatory role in the breakdown of glycogen. The two types of PhK deficiency are liver PhK deficiency (characterized by early childhood onset of hepatomegaly and growth restriction, and often, but not always, fasting ketosis and hypoglycemia) and muscle PhK deficiency, which is considerably rarer (characterized by any of the following: exercise intolerance, myalgia, muscle cramps, myoglobinuria, and progressive muscle weakness). While symptoms and biochemical abnormalities of liver PhK deficiency were thought to improve with age, it is becoming evident that affected individuals need to be monitored for long-term complications such as liver fibrosis and cirrhosis.

Cranioectodermal dysplasia (CED) is a ciliopathy with skeletal involvement (narrow thorax, shortened proximal limbs, syndactyly, polydactyly, brachydactyly), ectodermal features (widely spaced hypoplastic teeth, hypodontia, sparse hair, skin laxity, abnormal nails), joint laxity, growth deficiency, and characteristic facial features (frontal bossing, low-set simple ears, high forehead, telecanthus, epicanthal folds, full cheeks, everted lower lip). Most affected children develop nephronophthisis that often leads to end-stage kidney disease in infancy or childhood, a major cause of morbidity and mortality. Hepatic fibrosis and retinal dystrophy are also observed. Dolichocephaly, often secondary to sagittal craniosynostosis, is a primary manifestation that distinguishes CED from most other ciliopathies. Brain malformations and developmental delay may also occur.

Congenital bile acid synthesis defect-3 (CBAS3) is an autosomal recessive disorder characterized by prolonged jaundice after birth, hepatomegaly, conjugated hyperbilirubinemia, elevations in characteristic abnormal bile acids, and progressive intrahepatic cholestasis with liver fibrosis (summary by Setchell et al., 1998 and Ueki et al., 2008). For a general phenotypic description and a discussion of genetic heterogeneity of congenital bile acid synthesis defects, see 607765.

Meckel syndrome, also known as Meckel-Gruber syndrome, is a severe pleiotropic autosomal recessive developmental disorder caused by dysfunction of primary cilia during early embryogenesis. There is extensive clinical variability and controversy as to the minimum diagnostic criteria. Early reports, including that of Opitz and Howe (1969) and Wright et al. (1994), stated that the classic triad of Meckel syndrome comprises (1) cystic renal disease; (2) a central nervous system malformation, most commonly occipital encephalocele; and (3) polydactyly, most often postaxial. However, based on a study of 67 patients, Salonen (1984) concluded that the minimum diagnostic criteria are (1) cystic renal disease; (2) CNS malformation, and (3) hepatic abnormalities, including portal fibrosis or ductal proliferation. In a review of Meckel syndrome, Logan et al. (2011) stated that the classic triad first described by Meckel (1822) included occipital encephalocele, cystic kidneys, and fibrotic changes to the liver. Genetic Heterogeneity of Meckel Syndrome See also MKS2 (603194), caused by mutation in the TMEM216 gene (613277) on chromosome 11q12; MKS3 (607361), caused by mutation in the TMEM67 gene (609884) on chromosome 8q; MKS4 (611134), caused by mutation in the CEP290 gene (610142) on chromosome 12q; MKS5 (611561), caused by mutation in the RPGRIP1L gene (610937) on chromosome 16q12; MKS6 (612284), caused by mutation in the CC2D2A gene (612013) on chromosome 4p15; MKS7 (267010), caused by mutation in the NPHP3 (608002) gene on chromosome 3q22; MKS8 (613885), caused by mutation in the TCTN2 gene (613846) on chromosome 12q24; MKS9 (614209), caused by mutation in the B9D1 gene (614144) on chromosome 17p11; MKS10 (614175), caused by mutation in the B9D2 gene (611951) on chromosome 19q13; MKS11 (615397), caused by mutation in the TMEM231 gene (614949) on chromosome 16q23; MKS12 (616258), caused by mutation in the KIF14 gene (611279) on chromosome 1q32; MKS13 (617562), caused by mutation in the TMEM107 gene (616183) on chromosome 17p13; and MKS14 (619879), caused by mutation in the TXNDC15 gene (617778) on chromosome 5q31.

Any renal-hepatic-pancreatic dysplasia in which the cause of the disease is a mutation in the NPHP3 gene.

The nephronophthisis (NPH) phenotype is characterized by reduced renal concentrating ability, chronic tubulointerstitial nephritis, cystic renal disease, and progression to end-stage renal disease (ESRD) before age 30 years. Three age-based clinical subtypes are recognized: infantile, juvenile, and adolescent/adult. Infantile NPH can present in utero with oligohydramnios sequence (limb contractures, pulmonary hypoplasia, and facial dysmorphisms) or postnatally with renal manifestations that progress to ESRD before age 3 years. Juvenile NPH, the most prevalent subtype, typically presents with polydipsia and polyuria, growth retardation, chronic iron-resistant anemia, or other findings related to chronic kidney disease (CKD). Hypertension is typically absent due to salt wasting. ESRD develops at a median age of 13 years. Ultrasound findings are increased echogenicity, reduced corticomedullary differentiation, and renal cysts (in 50% of affected individuals). Histologic findings include tubulointerstitial fibrosis, thickened and disrupted tubular basement membrane, sporadic corticomedullary cysts, and normal or reduced kidney size. Adolescent/adult NPH is clinically similar to juvenile NPH, but ESRD develops at a median age of 19 years. Within a subtype, inter- and intrafamilial variability in rate of progression to ESRD is considerable. Approximately 80%-90% of individuals with the NPH phenotype have no extrarenal features (i.e., they have isolated NPH); ~10%-20% have extrarenal manifestations that constitute a recognizable syndrome (e.g., Joubert syndrome, Bardet-Biedl syndrome, Jeune syndrome and related skeletal disorders, Meckel-Gruber syndrome, Senior-Løken syndrome, Leber congenital amaurosis, COACH syndrome, and oculomotor apraxia, Cogan type).

Neonatal sclerosing cholangitis (NSC) is a rare autosomal recessive form of severe liver disease with onset in infancy. Affected infants have jaundice, cholestasis, acholic stools, and progressive liver dysfunction resulting in fibrosis and cirrhosis; most require liver transplantation in the first few decades of life. Cholangiography shows patent biliary ducts, but there are bile duct irregularities (summary by Girard et al., 2016; Grammatikopoulos et al., 2016).

Biliary atresia is a disorder of infants in which there is progressive obliteration or discontinuity of the extrahepatic biliary system, resulting in obstruction of bile flow. Untreated, the resulting cholestasis leads to progressive conjugated hyperbilirubinemia, cirrhosis, and hepatic failure (Bates et al., 1998). Most patients require liver transplantation within the first year of life (Leyva-Vega et al., 2010). See also Alagille syndrome (118450), which includes biliary atresia as a feature.

Short-rib thoracic dysplasia (SRTD) with or without polydactyly refers to a group of autosomal recessive skeletal ciliopathies that are characterized by a constricted thoracic cage, short ribs, shortened tubular bones, and a 'trident' appearance of the acetabular roof. SRTD encompasses Ellis-van Creveld syndrome (EVC) and the disorders previously designated as Jeune syndrome or asphyxiating thoracic dystrophy (ATD), short rib-polydactyly syndrome (SRPS), and Mainzer-Saldino syndrome (MZSDS). Polydactyly is variably present, and there is phenotypic overlap in the various forms of SRTDs, which differ by visceral malformation and metaphyseal appearance. Nonskeletal involvement can include cleft lip/palate as well as anomalies of major organs such as the brain, eye, heart, kidneys, liver, pancreas, intestines, and genitalia. Some forms of SRTD are lethal in the neonatal period due to respiratory insufficiency secondary to a severely restricted thoracic cage, whereas others are compatible with life (summary by Huber and Cormier-Daire, 2012 and Schmidts et al., 2013). There is phenotypic overlap with the cranioectodermal dysplasias (Sensenbrenner syndrome; see CED1, 218330). Genetic Heterogeneity of Asphyxiating Thoracic Dysplasia SRTD1 has been mapped to chromosome 15q13. See also SRTD2 (611263), caused by mutation in the IFT80 gene (611177); SRTD3 (613091), caused by mutation in the DYNC2H1 gene (603297); SRTD4 (613819), caused by mutation in the TTC21B gene (612014); SRTD5 (614376), caused by mutation in the WDR19 gene (608151); SRTD6 (263520), caused by mutation in the NEK1 gene (604588); SRTD7 (614091), caused by mutation in the WDR35 gene (613602); SRTD8 (615503), caused by mutation in the WDR60 gene (615462); SRTD9 (266920), caused by mutation in the IFT140 gene (614620); SRTD10 (615630), caused by mutation in the IFT172 gene (607386); SRTD11 (615633), caused by mutation in the WDR34 gene (613363); SRTD13 (616300), caused by mutation in the CEP120 gene (613446); SRTD14 (616546), caused by mutation in the KIAA0586 gene (610178); SRTD15 (617088), caused by mutation in the DYNC2LI1 gene (617083); SRTD16 (617102), caused by mutation in the IFT52 gene (617094); SRTD17 (617405), caused by mutation in the TCTEX1D2 gene (617353); SRTD18 (617866), caused by mutation in the IFT43 gene (614068); SRTD19 (617895), caused by mutation in the IFT81 gene (605489); SRTD20 (617925), caused by mutation in the INTU gene (610621); and SRTD21 (619479), caused by mutation in the KIAA0753 gene (617112). See also SRTD12 (Beemer-Langer syndrome; 269860).

Combined oxidative phosphorylation deficiency-37 is an autosomal recessive multisystem disorder apparent at birth or in the first months of life. Affected individuals have hypotonia, failure to thrive, and neurodegeneration with loss of developmental milestones, as well as liver dysfunction. Some patients may have hypertrophic cardiomyopathy, loss of vision and hearing, and/or seizures. Mitochondrial respiratory dysfunction is apparent in liver and skeletal muscle tissue. Most patients die in childhood (summary by Zeharia et al., 2016). For a discussion of genetic heterogeneity of combined oxidative phosphorylation deficiency, see COXPD1 (609060).

Rajab interstitial lung disease with brain calcifications-1 (RILDBC1) is an autosomal recessive multisystem disorder with a highly variable phenotype. Most patients present in infancy or early childhood with poor growth and interstitial lung disease, which may lead to death. Some may also have liver, skeletal, and renal abnormalities, and most have intracranial calcifications on brain imaging. Some may have early impaired motor development, but most have normal cognitive development (summary by Xu et al., 2018). Genetic Heterogeneity of Rajab Interstitial Lung Disease with Brain Calcifications Also see Rajab interstitial disease with brain calcifications-2 (RILDBC2; 619013), caused by mutation in the FARSA gene (602918).

Congenital disorder of glycosylation type IIw (CDG2W) is an autosomal dominant metabolic disorder characterized by liver dysfunction, coagulation deficiencies, and profound abnormalities in N-glycosylation of serum specific proteins. All reported patients carry the same mutation (602671.0017) (summary by Ng et al., 2021). For an overview of congenital disorders of glycosylation, see CDG1A (212065) and CDG2A (212066).

Biliary, renal, neurologic, and skeletal syndrome (BRENS) is an autosomal recessive complex ciliopathy with multisystemic manifestations. The most common presentation is severe neonatal cholestasis that progresses to liver fibrosis and cirrhosis. Most patients have additional clinical features suggestive of a ciliopathy, including postaxial polydactyly, hydrocephalus, retinal abnormalities, and situs inversus. Additional features of the syndrome may include congenital cardiac defects, echogenic kidneys with renal failure, ocular abnormalities, joint hyperextensibility, and dysmorphic facial features. Some patients have global developmental delay. Brain imaging typically shows dilated ventricles, hypomyelination, and white matter abnormalities, although some patients have been described with abnormal pituitary development (summary by Shaheen et al., 2020 and David et al., 2020).

Progressive familial intrahepatic cholestasis-8 (PFIC8) is an autosomal recessive disorder characterized by cholestasis and high gamma-glutamyltransferase presenting in the infantile period (summary by Unlusoy Aksu et al., 2019). For a general phenotypic description and a discussion of genetic heterogeneity of PFIC, see PFIC1 (211600).