FasL expression in day-5 cells. Day-5 cells were analyzed for FasL by indirect immunofluorescence using a specific murine antihuman FasL MoAb or an isotype control added after permeabilization and were stained with FITC-goat antimouse IgG1. After blocking with mouse IgG in PBS, the cells were incubated with PE-MoAb to human CD71 to identify ECFCs. The histogram shown represents three separate staining procedures with superimposed data: (1) population in lower left quadrant represents fluorescence produced by presence of indirect isotype control plus FITC-goat antimouse IgG1 and murine PE-IgG1; (2) gray represents the fluorescence produced by the presence of indirect isotype control plus FITC-goat antimouse IgG1 and specific PE-MoAb to human CD71; (3) dark black represents fluorescence produced by indirect MoAb to FasL stained with FITC-goat antimouse IgG1 and PE-MoAb to CD71. (A) CD71⁺ large cell population, representing 84% of all cells; 99% of the cells were CD71⁺ and 47% were FasL⁺. (B) Large cells bearing high intensity CD71⁺ from the same experiment, representing 79% of all cells. Among these cells, 100% were CD71⁺ and 64% were FasL⁺. The purity of the ECFCs was 65% ± 6%.