Secundum atrial septal defect (ASD) is a common congenital heart malformation that occurs as an isolated anomaly in 10% of individuals with congenital heart disease. Uncorrected ASD can cause pulmonary overcirculation, right heart volume overload, and premature death (summary by Benson et al., 1998). Genetic Heterogeneity of Atrial Septal Defect The ASD1 locus has been mapped to chromosome 5p. Other forms of atrial septal defect that are associated with other congenital heart disease but no conduction defects or noncardiac abnormalities include ASD2 (607941), caused by mutation in the GATA4 gene (600576), and ASD4 (611363), caused by mutation in the TBX20 gene (606061). ASD3 (614089) and ASD5 (612794), in which atrial septal defect is not associated with other cardiac abnormalities, are caused by mutation in the MYH6 (160710) and ACTC1 (102540) genes, respectively. ASD6 (613087), in which atrial septal defect may be associated with aneurysm of the interatrial septum and cardiac arrhythmias, is caused by mutation in the TLL1 gene (606742). ASD7 (108900), in which ASD is often associated with atrioventricular conduction defects, is caused by mutation in the NKX2-5 gene (600584). ASD8 (614433), in which ASD may be associated with other cardiac anomalies, is caused by mutation in the CITED2 gene (602937). ASD9 (614475) is caused by mutation in the GATA6 gene (601656). Somatic mutations in the HAND1 gene (602406) have been identified in tissue samples from patients with ASDs.