HM74 is a member of the G protein coupled receptor (GPCR) superfamily. In humans, HM74 is encoded by two different genes (GPR109A and GPR109B) that express two distinct proteins, namely HM74A and HM74B (also known as simply HM74), which are 96% homologous. In mice and rats, only one gene (deisngnated Gpr109a) encodes the HM74 protein. HM74B is a G(i) protein-coupled receptor that mediates the metabolic effects of nicotinic acid. Localizing to the cell membrane, HM74B is highly expressed in adipocytes, immune cells and spleen and, like all members of the GPCR superfamily, contains seven transmembrane domains. HM74B lacks the characteristic N-linked glycosylation sites that are present in other GPCR family members and also shows a more diverged amino acid sequence homology from most family members, implying different ligand specificity