ISL1 and aortic stenosis: Wholemount and histological analyses of Isl1 hypomorphic embryos at E12.5 (Fig. 1a, b) and E17.5 (Fig. 1c, d) revealed various degrees of cardiac outflow tract (OFT) septation abnormalities, including partial OFT septal defects with aortic stenosis and misalignment (Fig. 1a, middle panel, Fig. 1b) and persistent truncus arteriosus (PTA) (Fig. 1a, right panel, Fig. 1c, d).