Previous genome-wide association studies (GWAS) have not confirmed the association between polymorphisms in the MTHFR gene or other genes implicated in Hcy metabolism and schizophrenia (Yoshimi et al., 2010; Lencz et al., 2013; Ripke et al., 2013, 2014; Ivorra et al., 2014; Saito et al., 2014) or bipolar disorder (Sklar et al., 2011; Kuo et al., 2014; Mühleisen et al., 2014; Xu et al., 2014) risk. Here, MTHFR is linked to bipolar disorder.