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. 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 Rat. Recombinant lymphotactin has been shown to have chemotactic activity for lymphocytes, NK cells and splenocytes.