Imatinib mesylate (Gleevec, STI571), a first generation oral BCR-ABL kinase inhibitor, blocks the growth of BCR-ABL -transformed cells, and is highly effective in inducing disease remission in chronic phase CML patients (remission rate > 90%) [23, 24]. This evidence concerns the gene ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive.