TTC3 and familial atrioventricular septal defect: In addition, we identified a cluster that contained three contiguous genes mapped to HSA21: PIGP (DSCR5), TTC3, and DSCR3. A numerical simulation to compute the probability of finding a triplet on chromosome 21, assuming that gene expression and regulation are independent of localization, showed that the triplet found on HSA21 in the AVSD group was significant (p = 0.039) (Table S4).