Another interesting relationship was observed between methotrexate (MTX) resistance and Hsp70, since it was observed that the phosphorylation status of Hsc70—a member of the Hsp70 family—regulates the transport of this drug to the cells via the reduced folate carrier system and contributes to MTX resistance in murine leukemia L1210 cells [161]. This evidence concerns the gene HSPA4 and leukemia.