Compared to the general population, individuals with Down syndrome (DS) are at a 500-fold increased risk for complete atrioventricular septal defects (AVSDs), a severe form of congenital heart defect (CHD) that significantly impacts morbidity and mortality in this population. The purpose of this study is to identify environmental exposures and genetic variants on chromosome 21 that contribute to CHD susceptibility using a candidate gene approach in a DS population with complete AVSDs. Specifically, 1) candidate genes within the 10Mb DS heart critical region will be prioritized by characterizing transcript expression in human fetal and adult heart tissue, 2) our current DS with AVSD population will be expanded to facilitate epidemiological and association studies, and 3) single nucleotide polymorphisms in candidate genes will be genotyped to conduct an association study utilizing statistical methods developed for use with trisomic populations.