Patient HH25 had a c.804C > G mutation in the TBX5 gene, a silent mutation that is not expected to be the cause of the typical HOS clinical features (ASD, VSD, mitral valve insufficiency, dilatation of the left atrium and the pulmonary artery, preaxial polydactyly of the right thumb with duplication of the distal phalanx (presence of nails was observed on both the duplicated phalanges), distal implantation of the left thumb, mild hemithorax hypoplasia, upper-limb supination limited to the right side, and a sacralized transition vertebra; Figure 1D). This evidence concerns the gene TBX5 and atrial septal defect.