For instance, the combined presence of the C421A (rs2231142, Gln141Lys) SNP in the ABCG2 gene (encoding the Breast Resistance Cancer Protein, BCRP transporter) and the C3435T transition in ABCB1 (coding for P-glycoprotein) has been related to the occurrence of encephalopathy in children with ALL treated with MTX (Erdilyi et al., 2008). This evidence concerns the gene ABCB1 and Encephalopathy.