Human CXCL12 is expressed as five isoforms that differ only in the C-terminal tail. The gamma isoform of CXCL12, also known as SDF-1 gamma, is a 12 kDa, heparin-binding member of the CXC (or alpha) family of chemokines ,Mature SDF-1 molecules are not glycosylated and exhibit a typical three antiparallel beta -strand chemokine-like fold. N-terminal aa 1 8 form a receptor binding site, while aa 1 and 2 (LysPro) are involved in receptor activation. All SDF-1 isoforms can undergo proteolytic processing of the first two N-terminal amino acids by CD26, which is thought to create a reducedactivity chemokine. Human SDF-1 gamma is synthesized as a 119 amino acid (aa) precursor that contains a 21 aa signal sequence and a 98 aa mature region. Mature human SDF-1 gamma shares 99%, 97% and 98% aa identity with mouse, rat, and equine SDF-1 gamma, respectively. The unique Cterminal 26 aa of SDF-1 gamma are highly charged, including four BBXB (where B = basic and X = any aa) motifs, while the most prevalent form, SDF-1 alpha, has 4 unique Cterminal aa and binds heparin via the shared BBXB site more Nterminally located . The SDF1 gamma Cterminus binds heparin in secreted SDF-1 gamma, or targets the isoform to the nucleolus in the absence of a signal sequence. SDF-1 isoforms interact with CXCR4 and CXCR7 receptors on the cell surface, and can also bind syndecan-4.