Although the patient carried wild-type DPD and CDA genotypes, she also carried the 222Ala-429Glu and 222Val-429Glu diplotype (Table 1) associated with a higher likelihood of diarrhea and mucositis in patients receiving concurrent 5-FU and radiotherapy.8 The mechanism underlying this association is unclear, but may involve individual variation in MTHFR-mediated depletion of 5,10 methylenetetrahydrofolate, which is required for optimal thymidylate synthase inhibition. This evidence concerns the gene MTHFR and mucositis.