FUNCTION: Receptor for the C-X-C chemokine SDF-1. Transduces a signal by increasing the intracellular calcium ions level. Involved in haematopoiesis and in cardiac ventricular septum formation. Plays also an essential role in vascularization of the gastrointestinal tract, probably by regulating vascular branching and/or remodelling processes in endothelial cells. Could be involved in cerebellar development. In the CNS, could mediate hippocampal-neuron survival. Acts as a primary receptor for some HIV-2 isolates and as a coreceptor with CD4 for HIV-1 X4 viruses (lymphocyte-tropic HIV-1 viruses, also called syncytium-inducing (SI) strains). Promotes Env-mediated fusion of the virus.