In this report we describe the initial characterization of murine, human, and Drosophila hesr-1 (for hairy and enhancer of split related-1) a novel evolutionary conserved family of hairy/enhancer of split homologs. Hesr-1 cDNAs display features typical of hairy and enhancer of split-type bHLH proteins including a N-terminal bHLH domain a conserved orange domain immediately C-terminal to the bHLH region. Despite their similarity to known hairy/enhancer of split homologs, hesr-1 cDNAs are divergent members of the hairy and enhancer of split bHLH family since the degree of sequence identity within the bHLH and their nearest homologs are relatively low. Moreover, the tetrapeptide motif, WRPW, which is found in all hairy and enhancer of split family members, is not present in hesr-1 VSports手机版. Rather, a variant of this motif, YRPW, is found. Analysis of embryonic murine hesr-1 expression by in situ hybridization reveals strong expression in the somitic mesoderm, the central nervous system, the kidney, the heart, nasal epithelium, and limbs indicating a role for hesr-1 in the development of these tissues. Like the enhancer of split cDNAs in Drosophila, we show that hesr-1 expression depends critically on signaling through the notch pathway in murine embryos, suggesting that aspects of hesr-1 regulation and function might also be evolutionary conserved. .