Interleukin-24 (IL-24) also known as Melanoma differentiation-associated gene 7 protein (MDA-7) is a member of the IL10 family of cytokines. IL-24/MDA-7/IL24 can induce apoptosis selectively in various cancer cells. Overexpression of IL-24 leads to elevated expression of several GADD family genes, which correlates with the induction of apoptosis. The phosphorylation of mitogen-activated protein kinase 14 (MAPK7/P38), and heat shock 27kDa protein 1 (HSPB2/HSP27) are found to be induced by this gene in melanoma cells, but not in normal immortal melanocytes. Human IL-24/MDA-7/IL24 is secreted by activated peripheral blood mononuclear cells and is the ligand for two heterodimeric receptors, IL-22R1/IL-20R2 and IL-20R1/IL-20R2. Northern blot analysis revealed IL-24/MDA-7/IL24 expression in human tissues associated with the immune system such as the spleen, thymus, peripheral blood leukocytes, and normal melanocytes. IL-24/MDA-7/IL24 binding to either its endogenous receptors on human keratinocytes or to ectopically expressed receptors on baby hamster kidney cells leads to activation of the signal transducers and activators of transcription.