Chronic myeloid leukemia (CML), which is caused by the BCR–ABL fusion tyrosine kinase, is one of the most intensively studied human cancers. ABL kinase inhibitors have been spectacularly successful in treating CML, but disease persistence and acquired drug resistance can prevent eradication and cure of the leukemia. The development of better therapies will depend on a full understanding of signaling pathways in CML, facilitated by model studies using mutant mice.