Transcriptional repressor which binds preferentially to the canonical E box sequence 5'-CACGTG-3' (By similarity). Downstream effector of Notch signaling required for cardiovascular development. Specifically required for the Notch-induced endocardial epithelial to mesenchymal transition, which is itself criticial for cardiac valve and septum development. Represses transcription by the cardiac transcriptional activators GATA4 and GATA6. Interacts with HES1, HDAC1, NCOR1 and SIN3A (By similarity). Self-associates. Interacts with GATA4, GATA6, HEY1 and HEY2. Expressed in heart and at lower levels in brain, lung, muscle, ovary and testis. Expressed in the presmitic mesoderm, the somites, the developing peripheral nervous system and arterial smooth muscle. Expressed in atrioventricular cushions from 9.5 dpc to 12.5 dpc. Also expressed in developing retina. By activation of the Notch signaling pathway. Expression in nascent somites is reduced in mice lacking Notch signaling. Belongs to the HEY family.