Human lymphotactin (Lptn)/XCL1 (also named human SCM-1 alpha and ATAC) and its mouse homologue belong to the C or gamma subfamily of chemokines. The C chemokines lack two (the 1st and 3rd ) of the four invariant cysteine residues normally found in the CC and CXC chemokines and have an extended carboxy terminus. Human lymphotactin encodes a 114 amino acid residue precursor protein with a 21 amino acid residue predicted signal peptide. The expression of lymphotactin is abundant in some activated T cells such as activated CD8+ T cells and other class I MHC restricted T cells. Lptn expression is absent in CD4+ T cells. Human and mouse Lptn share approximately 60% amino acid sequence homology. The gene for lymphotactin has been mapped to chromosome 1 in both human and mouse. Recombinant human lymphotactin has been shown to have chemotactic activity for lymphocytes and NK cells. The orphan receptor GPR5 has been reported to be the specific receptor for Lptn.