Therefore, we investigate the potential role of candidate polymorphisms in SLCO1B1 (rs4149056 and rs2306283), ABCB1 (rs1045642), ABCC2 (rs717620), ABCC3 (rs9895420), and ABCC4 (rs7317112) on HD‐MTX PK and on patients' outcome (meant both as relapse and drug‐related toxicities during the consolidation phase) in an Italian cohort of ALL pediatric patients treated according to the AIEOP‐BFM ALL 2009 protocol. The gene discussed is SLCO1B1; the disease is acute lymphoblastic leukemia.