In one prospective study where TYMS genotyping was used to select the chemotherapy of choice in patients with rectal cancer, the rate of grade 3–4 FrAEs was less in patients with 2R/2R, 2R/3R, or 2R/4R TYMS genotypes compared to patients with 3R/3R or 3R/4R TYMS genotypes (30% vs 54%). Here, TYMS is linked to rectal cancer.