Indeed, numerous studies have investigated the impact of several SNPs in genes related to capecitabine’s PD, such as TYMS, enolase superfamily member 1 gene (ENOSF1), methylenetetrahydrofolate reductase gene (MTHFR), ERCC excision repair 1, endonuclease non-catalytic subunit gene (ERCC1), ERCC excision repair 2, TFIIH core complex helicase subunit gene (ERCC2), X-ray repair cross-complementing 1 gene (XRCC1), and X-ray repair cross-complementing 3 gene (XRCC3) on the effectiveness of FP therapy in gastrointestinal neoplasms [9,10,11,12,13,14,15,16,17,18]. The gene discussed is MTHFR; the disease is digestive system neoplasm.