Transcriptional repressor which functions as a downstream effector of Notch signaling in 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 HEY1 to specify arterial cell fate or identity. Promotes maintenance of neuronal precursor cells and glial versus neuronal fate specification. Binds preferentially to the canonical E box sequence 5'-CACGTG-3'. 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 ARNT (By similarity). Interacts with GATA4, GATA6, HES1 and HEYL. Interacts with HDAC1, NCOR1 and SIN3A. Highly expressed in the aorta, lower expression detected in the heart, brain, kidney, lung, muscle, ovary and testis. Expressed in the developing somites and the ventricles of the heart. Expressed in the otic vesicles between 8.5 dpc and 10.5 dpc. Expressed in the myocardium of the ventricles at 9.5 dpc and in the atrioventricular cushions from 9.5 to 12.5 dpc. At 10.5 dpc, strongly expressed in the spinal nerves, the cranial ganglia and the telencephalon. At 11.5 dpc, expressed in the craniofacial region of the distal part of the maxillary arch, along the rostral mandibular arch and surrounding the lateral nasal processes. Expressed in the midbrain-hindbrain boundary and the posterior edge of the hand- and foot-paddle. Expressed in the mediodorsal region of the telencephalon and the ventricular zone of the ventral spinal cord at 12 dpc, then in the ventral region of the telencephalon and the cortical plate at 15 dpc. Expression in the heart is limited to the compact myocardial layer at 17.5 dpc. Also expressed in the developing retina up to P5, at which point expression decreases. By activation of the Notch signaling pathway. Mice display a spectrum of cardiac malformations including ventricular septal defects, tetralogy of Fallot and tricuspid atresia. The penetrance of the cardiac malformation phenotype varies according to the strain, suggesting the presence of modifier genes. Belongs to the HEY family.