The 677C → T polymorphism in MTHFR was examined in persons with schizophrenia, major depression, and bipolar disorder.85 The TT variant was found in 12% of 419 control subjects, 21% of 297 patients with schizophrenia (p < 0.0006; p < 0.002 after Bonferroni correction), 28% of 32 patients with major depression (p < 0.06; p < 0.02 after Bonferroni correction), and 13% of 40 patients with bipolar disorder (NS). Here, MTHFR is linked to major depressive disorder.