As for TYMS, it is considered the primary site of action for 5-fluorodeoxyuridine monophosphate (5-FdUMP), one of the three metabolites of 5-FU and, along with 5,10-methylene tetrahydrofolate, forms a ternary complex that blocks the access of dUMP to the nucleotide-binding site of TYMS by competition with FdUMP, resulting in a deoxynucleotides imbalance, such as deoxyuridine triphosphate (dUTP), that leads to DNA damage in cancer cells [1,60]. The gene discussed is TYMS; the disease is cancer.