BCR and leukemia: In addition, the presence of genetic alterations typical of leukemia cells of some ALL patients (e.g., ETV6-RUNX1, TCF3-PBX1, KMT2A, and BCR-ABL) on in vitro expanded MSC, together with the presence of the BCR-ABL transcript fusion in BM-derived EC from CML patients, point out the close relationship between leukemia cells and other cells in the BM microenvironment [14,15,16].