MTHFR and schizophrenia: Roffman et al. reported that the MTHFR T allele undermines category generation in schizophrenia, with 33% of T/T patients unable to complete a single category, and then reported that COMT Val108/158Met does not significantly influence this aspect of executive function, by itself or in interaction with MTHFR C677T [35, 37].