Studies of individual with both SNPs (C677T and A1298C) showed that subjects with heterozygosity for both mutations resulted in an even lower MTHFR activity than heterozygosity for single MTHFR mutations, while no subjects carry both homozygote for MTHFR mutations regardless which SNPs15 Furthermore, There were studies of multiple polymorphisms of one-carbon metabolism and schizophrenia symptoms showed an increase negative symptoms severity with increase of risk alleles, suggesting a cumulative effects of risk SNPs in one-carbon metabolism61. The gene discussed is MTHFR; the disease is schizophrenia.