The landmark event in the revolution of targeted therapy was represented by the development in the early '90s of the first selective tyrosine kinase inhibitor, Imatinib mesylate (Glivec®), a specific competitive inhibitor of ATP, directed to the fusion protein BCR-ABL typical of patients with chronic myelogenous leukemia (CML) and ALL that are Philadelphia chromosome-positive (Ph+). This evidence concerns the gene ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive.