In the present study, correlations were identified between the polymorphism patterns of TS, MTHFR, ERCC1, ERCC2, GSTP1, GSTT1 and GSTM1 and the clinical outcome, including the incidence of peripheral neuropathy, in Japanese MCRC patients who were treated with modified FOLFOX6 (mFOLFOX6). The gene discussed is MTHFR; the disease is peripheral neuropathy.