Our main objective was to determine the effect of dietary ascorbate supplementation on the development of tumors in mice unable to synthesize ascorbic acid, gulonolactone oxidase (gulo) knockout (KO) mice using two models: challenging mice with breast cancer 4T1 cells into the mammary pads and challenging with injection of melanoma B16FO cancer cells intraperitoneally (IP). The gene discussed is GULOP; the disease is breast cancer.