Mouse lymphotactin (Lptn) and its human homologue (also named human SCM-1 and ATAC) 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. Mouse lymphotactin encodes a 114 amino acid residue precursor protein with a 21 amino acid residue predicted signal peptide. The expression of lymphotactin is restricted to activated mouse pro-T cells and to activated, class I MHC restricted T cells. The gene for lymphotactin has been mapped to chromosome 1 in both human and mouse. Recombinant mouse lymphotactin has been shown to have chemotactic activity for lymphocytes, NK cells and mouse splenocytes.