The stromal cell derived factor 1 (SDF1, CXCL12) is a small, pro-inflammatory chemoattractant cytokine that regulates leukocyte trafficking through interactions with its cognate 7-transmembrane G protein-coupled receptors. The SDF1 receptor, CXCR4, also serves as a coreceptor for the entry of human immunodeficiency virus into target cells. SDF1 may regulate homing and maintenance of CXCR4-expressing stem or progenitor cells, including embryonic and many somatic stem cells. Many cancer cells express CXCR4, suggesting that SDF1 plays a role in cancer metastasis. Alternative splicing and differential processing during maturation produce a pair of SDF1 isoforms (SDF1α and SDF1β) that have different properties and biological activities. Additional isoforms of SDF1 have been reported.