Interleukin 16 (IL-16), also known as lymphocyte chemoattractant factor (LCF), refers to the C-terminal peptide of pro-IL-16 isoform 1, which is a nuclear and cytoplasmic PDZ-containing protein. The C-terminus of pro-IL-16 is cleaved by Caspase-3, resulting in the release of secreted IL-16. Mature IL-16 has been shown to bind the cell surface receptor CD4 and functions as an immunomodulatory cytokine.