GWAS studies conducted on ALL pediatric patients identified SNPs in the SLCO1B1 gene, encoding for the OAT1B1 MTX‐influx transporter and localized in the sinusoidal membrane of hepatocytes, as the major genetic marker of HD‐MTX pharmacokinetic variability [15, 48]. The gene discussed is SLCO1B1; the disease is acute lymphoblastic leukemia.