CXCR4, also known as fusin or LESTR, is a G protein-coupled receptor (GPCR) member of the CXC subfamily of chemokine receptors. Human CXCR4 is 352 amino acids (aa) in length with a predicted molecular weight of 40 kDa. The mouse and rat proteins share 91% aa sequence identity with the human protein. CXCR4 is the chemokine receptor for CXCL12. CXCR4 is a critical regulator of progenitor and stem cell mobilization and recruitment during development and during hematopoiesis. As such, antagonists of CXCR4 can be used to harvest hematopoietic stem/progenitor cells from bone marrow. Significantly, CXCR4 also acts as a co-receptor for HIV and mediates viral entry and infection of T cells. Studies on multiple tumor types implicate the CXCL12:CXCR4 axis in tumor metastasis and angiogenesis.