Transcriptional repressor which binds preferentially to the canonical E box sequence 5'-CACGTG-3'. 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. May be required in conjunction with HEY2 to specify arterial cell fate or identity. Promotes maintenance of neuronal precursor cells and glial versus neuronal fate specification. Represses transcription by the cardiac transcriptional activators GATA4 and GATA6 and by the neuronal bHLH factors ASCL1/MASH1 and NEUROD4/MATH3. May self-associate (By similarity). Interacts with HES1, NCOR1 and SIN3A (By similarity). Interacts with GATA4, GATA6 and HDAC1 and HEYL. Interacts with CCDC89/BOIP. Expressed in somitic mesoderm, brain, central nervous system, kidney, heart, nasal epithelium, limbs, lung, muscle, ovary and testis. Expressed in the developing nervous system, the somites, heart and craniofacial region. Expressed in the myocardium of the atrium at 9.5 dpc and in the atrioventricular cushions from 9.5 dpc to 12.5 dpc. Also expressed in developing and adult retina. Expressed in the ventricular zone of the ventral spinal cord at 12 dpc and in the ventricular zone of the telencephalon at 12 dpc and 15 dpc. Weakly expressed in the cortical plate at 15 dpc. By activation of the Notch signaling pathway. Belongs to the HEY family.