Bone marrow, 4T1 breast carcinoma, and NXS2 neuroblastoma cells contain a side population (SP). (A-C) The cell types indicated were stained with Hoechst 33342 dye to determine the presence of a Hoechst effluxing SP. (D-F) Cells were stained with Hoechst dye in the presence of the Ca⁺⁺-channel blocker verapamil hydrochloride to verify the specificity of the SP population. Each figure represents 1 of 2 experiments.