This case study illustrates how the determination of multiple co-crystal structures of the protein tyrosine kinase c-Abl was used to support drug discovery efforts leading to the design of nilotinib, a newly approved therapy for imatinib-intolerant and – resistant chronic myelogenous leukemia. Chronic myelogenous leukemia (CML) results from the BCR-Abl onco-protein, which possesses a constitutively activated Abl tyrosine kinase domain. Although many chronic-phase CML patients treated with imatinib as first-line therapy maintain excellent, durable responses, patients who have progressed to advanced-stage CML frequently fail, or lose their response to therapy, often due to the emergence of drug-resistant mutants of the protein. More than 60 such point mutations have been detected in imatinib-resistant patients. We determined the crystal structures of wild-type and mutant Abl kinase in complex with imatinib and other small molecule Abl inhibitors, with the aim of understanding the molecular basis for resistance and to aid in the design and optimization of inhibitors active against the resistance mutants. These results are presented in a way which illustrates the approaches used to generate multiple structures, the type of information that can be gained and the way this information is used to support drug discovery.