In order to tackle these issues, we aimed to investigated genetic variants in pharmacogenes involved in the FMP, DHFR, TYMS, MTHFR, SLC19A1 and SLCO1B1, and their association with MTX pharmacokinetics and toxicity in the consolidation phase of treatment of pediatric ALL patients in the context of the Serbian population, making this study the first of its kind. The gene discussed is SLCO1B1; the disease is acute lymphoblastic leukemia.