ARL4D and familial atrioventricular septal defect: A second male with a partial AVSD exhibited a large de novo deletion of 1.5 Mbp at 20p12.3 containing three protein coding genes; HAO1, TMX4, PLCB1. Only one of 50 genotyped DS patients had a small 163 kbp duplication at 17q21.31 of inherited from an unaffected mother containing one protein coding gene ARL4D (Table I).