Hes genes code nuclear proteins that suppress transcription. This protein belongs to the basic helix-loop-helix (bHLH) family of transcription factors. Hes genes are mammalian homologs of the Drosophila genes hairy and Enhancer of split [E(spl)]. There are seven members in the Hes family (Hes1-7), although Hes4 is absent in the mouse genome (Table 1). Hes genes encode nuclear proteins that repress transcription both actively and passively.