Munc13 proteins constitute a family of three highly homologous molecules (Munc13-1, Munc13-2 and Munc13-3) with homology to Caenorhabditis elegans unc-13p. Munc13 proteins contain a phorbol ester-binding C1 domain and two C2 domains, which are Ca2+/phospholipid binding domains. Sequence analyses have uncovered two regions called Munc13 homology domains 1 (MHD1) and 2 (MHD2) that are arranged between two flanking C2 domains. MHD1 and MHD2 domains are present in a wide variety of proteins from Arabidopsis thaliana, C. elegans, Drosophila melanogaster, mouse, rat and human, some of which may function in a Munc13-like manner to regulate membrane trafficking. The MHD1 and MHD2 domains are predicted to be alpha-helical.