Promoters of Hey1, Hey and Heyl genes contain conserved RBPJ (CSL) binding sites. Cotransfection experiments in mouse fibroblast cell line NIH3T3, human embryonic kidney cell line HEK293 and African green monkey kidney cell line COS-7 showed that a recombinant mouse NICD1 activates transcription of a reporter luciferase gene fused to promoters of either mouse Hey1, Hey2 or Heyl. NICD1-mediated transcriptional activation is lost when RBPJ sites in Hey1 promoter are deleted by PCR-directed mutagenesis (Maier and Gessler 2000). Combined loss of Hey1 and Hey2 results in embryonic death of mouse embryos after embryonic day 9.5 with a global lack of vascular remodeling and massive hemorrhage. Similar vascular defects are observed in Jag1 and Notch1 knockout mice, implicating Hey1 and Hey2 as essential transducers of Notch signals in cardiovascular development and in mediation of arterial cell fate decisions (Fischer et al. 2004). Heyl is strongly expressed in the presomitic mesoderm, somites, peripheral nervous system and arterial smooth muscles of mouse embryos. Heyl expression in nascent somites is lost in Notch1 and Dll1 knockout mice, implicating Heyl as a Notch effector during somite formation (Leimeister et al. 2000).