CXCR4 is the receptor for the CXC chemokine SDF1 that has essential functions on embryo organogenesis, immunological functions and T lymphocyte trafficking. CXCR4 is the only SDF1 receptor identified so far. This suggests that CXCR4 expression is critical for the biological effects of SDF1. CXCR4 is a seven-transmembrane-spanning, G-protein-coupled receptor for the CXC chemokine PBSF/SDF-1, it functions as a co-receptor for T-cell-line tropic human immunodeficiency virus HIV-1. Tachibana et al. (1998) conclude that PBSF/SDF-1 and CXCR4 define a new signalling system for organ vascularization.