Reticulocyte isolation by flow cytometry. Peripheral blood red cells were dual stained with anti–transferrin receptor (CD71) and a RNA-intercalating dye, thiazole orange (TO). (A) The CD71 fluorescence pattern of the erythrocyte population in peripheral blood. The forward scatter (size) is shown on the x-axis, and the box shows the CD71⁺ cells. (B) The TO fluorescence of the CD71⁺ cells in panel A; 97.5% of the CD71⁺ cells were also positive for TO.