CD184, also known as CXCR4 or fusin, is a receptor for the C-X-C chemokine SDF-1. It is expressed mainly in hematopoietic cells, vascular endothelium, and neural tissue. CD184 is a G-protein coupled receptor containing extracellular N-terminal, seven transmembrane domains and intracellular C-terminal domain. It transduces signal by increasing the intracellular calcium level. CD184 plays an essential role in vascularization of the gastrointestinal tract, and is involved in cerebellar development and in hematopoiesis. It is also a coreceptor (with CD4) for HIV-1 X4 virus and a primary receptor for some HIV-2 isolates.