Since MTHFR enzymatic deficiency may theoretically favour thymidine synthesis via an increase of CH2FH4, one can hypothesise that tumours exhibiting mutated MTHFR genotypes linked to enzymatic deficiency may be more sensitive to FU cytotoxicity than wild-type (wt) MTHFR genotype tumours. The gene discussed is MTHFR; the disease is neoplasm.