Fig. 1. Structure and function of Hes factors. (A) The conserved domains of Hes factors. The basic (blue), HLH (mauve), Orange (orange) and WRPW (pink) domains and their functions are indicated. (B) Active repression: Hes factors bind to the N box or class C site by forming homodimers (left panel) or heterodimers with Hey (right panel) and actively repress transcription by interacting with co-repressors, such as Groucho homologs. (C) Passive repression: Hes factors form non-DNA-binding heterodimers with bHLH activators such as E47 and inhibit transcriptional activation. (D) Activation: bHLH activators such as Mash1 and E47 form heterodimers that bind to the E box and activate transcription.