Level of drug efflux and response to single-agent gemtuzumab ozogamicin in pediatric patients with AML. This figure demonstrates the response (defined as CR or CRp) for patients whose leukemic blasts showed an efflux ratio less than 1.2 compared with a ratio of 1.2 or greater (P = .005). The ratio of drug efflux is defined in “Patients, materials, and methods.”