Flow cytometry analysis of mouse fetal liver cells. Mouse fetal liver cells were freshly isolated from E14.5 embryos and double labeled with a FITC-conjugated anti-CD71 monoclonal antibody (mAb) and a PE-conjugated anti-TER119 mAb. Dead cells (propidium iodide–positive) and debris (low forward scatter) were excluded from analysis. The top left panel illustrates a density plot of all viable fetal liver cells; axes indicate relative logarithmic fluorescence units for PE (x-axis) and FITC (y-axis). Regions R1 to R5 are defined by characteristic staining pattern of cells, including CD71^medTER119^low, CD71^highTER119^low, CD71^highTER119^high, CD71^medTER119^high, and CD71^lowTER119^high, respectively. R1 to R5 cells were sorted by FACS and their purity was reanalyzed, as shown in the remaining 5 panels.