Interleukin-16, encoded by the IL16 gene, is a pleiotropic cytokine that functions as a chemoattractant, a modulator of T cell activation, and an inhibitor of HIV replication. As a ligand for CD4, the signaling process of this cytokine is mediated by CD4. It stimulates a migratory response in CD4+ lymphocytes, monocytes, and eosinophils as well as induces T-lymphocyte expression of interleukin 2 receptor. The product of this gene undergoes proteolytic processing, which yields two functional proteins. The cytokine function is exclusively attributed to the secreted C-terminal peptide, while the N-terminal product may play a role in cell cycle control. Caspase 3 is involved in the proteolytic processing of this protein. Two alternatively spliced transcript variants encoding distinct isoforms have been reported. (From LocusLink, Swiss-Prot and NCI)