Trpm4 is involved in the determination of murine heart size, potentially through a regulation of myocyte proliferation during fetal development.27 In a patient with atrioventricular septal defect, mild left ventricular hypoplasia, and extracardiac features, we identified a frameshift insertion p.(Lys615Ilefs*49) in SMARCC1, a highly constrained gene encoding a core subunit of the SWI/SNF chromatin remodeling complex. Here, SMARCA1 is linked to familial atrioventricular septal defect.