We investigated 47 haplotype-tagging and three functional SNPs in four genes coding for enzymes involved in methotrexate intracellular metabolism: folypolyglutamate synthase (FPGS); gamma-glutamyl hydrolase (GGH); MTHFR; and 5-aminoimidazole-4-carboxamide ribonucleotide transformylase (ATIC); (Fig. 1) in a large cohort of patients with psoriasis treated with methotrexate. Here, MTHFR is linked to psoriasis.