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. Self-associates. Interacts with HES1 and HEYL. Interacts with HDAC1, NCOR1 and SIN3A. Interacts with GATA4 and GATA6. Interacts with CCDC89/BOIP. Belongs to the HEY family.