Pro-interleukin-16 is a protein that in humans is encoded by the IL16 gene. Interleukin 16 (IL-16) is a cytokine that is released by a variety of cells (including lymphocytes and some epithelial cells) that has been characterized as a chemoattractant for certain immune cells expressing the cell surface molecule CD4. IL-16 was originally described as a factor that could attract activated T cells in humans, it was previously called lymphocyte chemoattractant factor (LCF). Since then, this interleukin has been shown to recruit and activate many other cells expressing the CD4 molecule, including monocytes, eosinophils, and dendritic cells. The structure of IL-16 was determined following its cloning in 1994. This cytokine is produced as a precursor peptide (pro-IL-16) that requires processing by an enzyme called caspase-3 to become active. CD4 is the cell signaling receptor for mature IL-16.