Using this 5'-UTR polymorphism as a heterozygous marker, we have reported that there is a high incidence of loss of heterozygosity (LOH) at the TS locus in colorectal cancer tissue, and have demonstrated that the TS genotype in cancer tissue modulated by LOH to give either a 2R/loss or 3R/loss situation influences the chemosensitivity to 5-FU-related drugs and the prognosis of colorectal cancer patients [13]. The gene discussed is TYMS; the disease is cancer.