Mammalian bombesin (Bn) receptors comprise 3 subtypes: BB1, BB2, BB3. BB1 and BB2 are activated by the endogenous ligands gastrin-releasing peptide (GRP, P07492) (GRP), neuromedin B (NMB, P08949) (NMB) and GRP-(18-27). The three Bombesin receptor subtypes couple primarily to the Gq/11 and G12/13 family of G proteins.