The melanoma differentiation-associated gene-7 (mda-7) was cloned by subtraction hybridization as a molecule whose expression is elevated in terminally differentiated human melanoma cells. Current information based on structural and sequence homology, has led to the recognition of MDA-7 as an IL-10 family cytokine member and its renaming as IL-24. Northern blot analysis revealed mda-7/IL-24 expression in human tissues associated with the immune system such as spleen, thymus, peripheral blood leukocytes and normal melanocytes. The MDA-7/IL-24 mouse counterpart, FISP, appears to be a Th2-specific protein and the rat counterpart, C49A/MOB-5, is associated with wound healing and is also induced as a consequence of ras-transformation. A notable property of MDA-7/IL-24 is its ability to induce apoptosis in a large spectrum of human cancer derived cell lines, in mouse xenografts and upon intratumoral injection in human tumors (phase I clinical trials). Various aspects of this intriguing molecule including its cytokine and anti-tumoral effects are described and discussed.