Transcriptional repressor which binds preferentially to the canonical E box sequence 5'-CACGCG-3'. Required for the development of GABAergic neurons. Self-associates. Interacts with HES5 and HEY2. Expressed in heart and testis. Expressed in the progenitor domains for mesencephalic GABAergic neurons. Death between 2 and 5 weeks of age. The mesencephalic GABAergic progenitors in these animals fail to develop into neurons. Belongs to the HEY family.