Par-4 is a naturally occurring tumor suppressor. Studies have indicated that overexpression of Par-4 selectively induces apoptosis in cancer cells while leaving normal, health, cells unaffected. Mechanisms contributing to this cancer-selective action of Par-4 have been associated with PKA activation of intracellular Par-4 in cancer cells or GRP78 expression primarily on the surface of cancer cells. On the other hand, endogenous Par-4 sensitizes cells to the action of a broad range of apoptotic inducers acting via the extrinsic and intrinsic pathways. A number of binding partners of Par-4 have been identified and shown to regulate Par-4 function in cancer and other diseases, such as Alzheimer's and major depression. Recent studies have recognized a number of natural products, dietary supplements, synthetic molecules and FDA-approved drugs that induce the secretion of Par-4 protein to cause apoptosis in primary or metastatic tumors, one of which is in clinical trials. More than 50 different laboratories worldwide are involved in Par-4 based research of this unique protein that has progressed from the bench to clinical trials.