Genetic variations of genes encoding pivotal regulator and transport enzymes of the folate cycle (e.g. MTHFD1, MTRR, MTR, MTHFR, DHFR, GGH, SLCO1B1) could influence the available folate in the cells, and therefore might alter the susceptibility to ALL and the response to the therapy [7]–[12]. The gene discussed is MTHFR; the disease is acute lymphoblastic leukemia.