MTHFR and systemic sclerosis: As demonstrated pharmacogenetic markers of response to initial therapy with MTX and maintenance therapy with AZA in other diseases with proven roles in action and clearance of these medicaments [21,24,25,37,38,39], variants in TPMT, MTHFR and SLCO1B1 genes (rs1800460, rs1142345, rs1801133 and rs4149056 respectively) were selected for investigation in patients with SSc.