TBX5 heterozygous loss-of-function mutations underlie Holt-Oram syndrome (HOS), an autosomal dominant developmental disorder characterized by skeletal abnormalities and, frequently, cardiac defects including atrial septal defects (ASDs), ventricular septal defects (VSDs), and conduction defects (Basson et al. 1994, 1997; Newbury-Ecob et al. 1996; Li et al. 1997). The gene discussed is TBX5; the disease is Holt-Oram syndrome.