Human immunodeficiency virus (HIV) and related viruses require co-receptors to infect target cells. Some G protein coupled receptors including CCR5, CXCR4, CCR3,CCR2b and CCR8 in the chemokine receptor family, and four new human molecules GPR15, STRL33, GPR1 and V28 were recently identified as HIV coreceptors. The genes encoding human and monkey GPR15/BOB (for G protein-coupled receptor 15 and brother of Bonzo, respectively) were recently cloned. This novel G protein-coupled receptor serves as coreceptor for simian immunodeficiency virus (SIV), and for strains of HIV-2 and M-tropic HIV-1. The ligand for GPR15 has not been identified yet.