One hallmark of neoplasia is abnormal differentiation. Induction of differentiation, by chemical or biological methods, provides a possible therapeutic intervention. "Differentiation therapy" is well documented in several model systems. These include melanoma, in which treatment with interferon-beta and the protein kinase C activator mezerein induces irreversible growth arrest and terminal differentiation culminating in programmed cell death. Subtraction hybridization between terminally differentiated and untreated melanoma cells identified melanoma differentiation-associated gene-7 (mda-7), which is selectively induced during the process of melanoma terminal differentiation. Since its identification seven years ago, mda-7 has been the object of intense focus because of its unique biological properties. Firstly, mda-7 is a secreted protein having cytokine-like properties and belonging to the IL-10 cytokine family. Based on this consideration, mda-7 was renamed IL-24. Secondly if delivered by means of an adenoviral vector, mda-7 induces selective apoptosis in cancer cells of diverse origin, while sparing their normal cellular counterparts. As such, mda-7 has become a novel tool for cancer gene therapy and is currently undergoing phase II clinical trials to determine its clinical efficacy in patients. The present review examines the biological properties of mda-7 and the signaling pathways that contribute to its unique cancer-specific apoptosis-inducing properties.