By FACS analysis, ES-derived neutrophils express the granulocyte-specific antigens, Gr-1, and the mouse neutrophil-specific antigen. For FACS analysis, 100 000 bone marrow (BM)– and ES-derived neutrophils were stained per well as described in “Materials and methods.” Panels A and C are of purified mature mouse bone marrow–derived neutrophils, while panels B and D are of ES-derived neutrophils harvested from day-9 cultures in the neutrophil differentiation mix. Histograms of FACS staining for Gr-1 and the neutrophil-specific antigens (Ag) are shown in A-B and C-D, respectively.