Pharmacogenomic factors, such as SNPs in MTX efflux transporter genes ABCB1 [27] and ABCC4 [8], the organic anion transporter gene SLCO1B1 [17, 18], the MTX influx transporter SLC19A1 [28], and MTX metabolism enzymes like 5,10‐methylenetetrahydrofolate reductase (MTHFR) [27], may influence MTX clearance and serve as risk predictors for MTX‐induced mucositis. The gene discussed is ABCB1; the disease is mucositis.