Therapeutic benefits of olive leaf extract in cancer cells include: reducing mitochondrial membrane potential and function; increasing mitochondrial reactive oxygen species (ROS), which mediate pro-oxidant effects on cancer cells and represent a major anti-cancer mechanism causing cell cycle arrest, apoptosis induction, and cytotoxicity in cancer cells; upregulating pro-apoptotic proteins by cleaving poly (ADP-ribose) polymerase (PARP) and activating caspase 9 cascade leading to cancer cell apoptosis. This evidence concerns the gene CASP9 and cancer.