An illustrative historic example is that in chronic myelogenous leukemia which is initially sensitive to treatment with imatinib mesylate (targets the BCR-ABL) treatment resistance may emerge due to the acquisition of secondary mutations in BCR-ABL that are resistant to imatinib but that are sensitive to second generation BCR-ABL inhibitors such as dasatinib or nilotinib. This evidence concerns the gene ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive.