 SP cells can rapidly efflux lipophilic fluorescent dyes to produce a characteristic profile based on fluorescence-activated flow cytometric analysis.Previous studies have demonstrated SP cells in bone marrow obtained from patients with acute myeloid leukemia, suggesting that these cells might be candidate leukemic stem cells, and recent studies have found a SP of tumor progenitor cells in human solid tumors.These new data indicate that the ability of malignant SP cells to expel anticancer drugs may directly improve their survival and sustain their clonogenicity during exposure to cytostatic drugs, allowing disease recurrence when therapy is withdrawn.