Methylmalonic aciduria and homocystinuria of the cblL type (MAHCL) is an autosomal recessive metabolic disorder with onset of symptoms in infancy. The disorder is characterized by neurologic features, including seizures and profoundly impaired neurodevelopment. In the single reported patient, metabolic workup showed mild methylmalonicaciduria without homocystinuria, but complementation studies were consistent with a biochemical diagnosis of cblC (277400) and cblX (309541). Mutations in the MMACHC gene (609831) and HCFC1 (300019) were excluded before identification of mutation in the THAP11 gene (Quintana et al., 2017). The biochemical abnormalities in cblC and cblX tend to be mild, and some patients with cblX without homocystinuria have been reported (Yu et al., 2013). Since THAP11 forms a functional complex with HCFC1, it is possible that future reported patients with THAP11 mutations may have homocystinuria (see review by Watkins and Rosenblatt, 2022). [from OMIM]