Imatinib, or Gleevec, is a targeted anti-cancer drug that can keep chronic myelogenous leukemia (CML) in check for most patients for many years. Infrequently, relapse can occur. Doctors believe that suboptimal dosing can cause resistance. It is important for patients to take imatinib as prescribed by their doctor to fight the disease and to guard against resistance. Regularly scheduled monitoring helps doctors and patients know if the medicine is working and if patients are reaching treatment goals.