Beta-Carotene
Beta-Carotene is a naturally-occurring retinol (vitamin A) precursor obtained from certain fruits and vegetables with potential antineoplastic and chemopreventive activities.  As an anti-oxidant, beta carotene inhibits free-radical damage to DNA.  This agent also induces cell differentiation and apoptosis of some tumor cell types, particularly in early stages of tumorigenesis, and enhances immune system activity by stimulating the release of natural killer cells, lymphocytes, and monocytes. (NCI04)