HEY2 and Abnormal cardiac septum morphology: 108 Somatic mutations were identified in NKX2-5 and its molecular partners, TBX5 and GATA4, as well as the transcription factor HEY2, in formalin-fixed tissues taken from a collection of hearts with atrial septal defect, 109 ventricular septal defect, and atrioventricular canal defect.