The 12G5 antibody reacts with CD184 (CXCR4 or fusin), an ~45 kDa seven transmembrane G-protein-coupled receptor expressed on the surface of a broad range of cells, including T and B cells, dendritic cells, monocytes, macrophages, granulocytes, platelets, endothelial and epithelial cells, astrocytes and neurons. At least two isoforms of CD184 are generated by alternate mRNA splicing. Ligands for CD184 include the CXC chemokine SDF-1 and ubiquitin. CD184 has functional roles in mediating chemotaxis, B cell lymphopoiesis and myelopoiesis, cardiogenesis, vascularization and neuronal guidance. CD184 acts as a co-receptor for infection by some T cell-tropic isolates of HIV-1 and as an alternative receptor for CD4-independent infection by HIV-2; functions that are blocked by binding of the 12G5 antibody. The antibody also partially inhibits SDF-1-induced chemotaxis and calcium influx. In differentiation studies, CD184 is used along with other markers such as CD117 to monitor the emergence of definitive endoderm.