Indeed, it has been demonstrated that the sensitivity to 5-FU is affected by polymorphisms in the orotate phosphoribosyltransferase gene (OPRT, transforming 5-FU in 5-fluorouridine monophosphate) and, in cancer tissues, by the level of activity of the OPRT enzyme and by the OPRT/DPD activities ratio. The gene discussed is UMPS; the disease is cancer.