A member of the CXC chemokine family, CXCL12 is strongly chemotactic for T-, B-, and cancer cells. Known to exert its effects mainly through binding to the CXCR4 receptor, CXCL12 plays an important role in morphogenesis, angiogenesis, and cancer metastasis. CXCL12 also binds to atypical chemokine receptor ACKR3, which activates the beta-arrestin pathway and acts as a scavenger receptor for CXCL12. Encoded by a single gene, SDF-1 alpha (89 amino acids) and SDF-1 beta (93 amino acids) are derivatives of an alternative splicing event. SDF-1_ and SDF-1_ have identical amino acid sequences, differing only, by the presence of four extra amino acids at the C-terminus of the _ form. SDF-1 beta is constitutively expressed by stromal cells.