Late development of embryonic thymus in MafB-deficient mice. (A,B) Frozen sections of embryonic thymus isolated from indicated mice at E18.5 were multicolor-stained either for AIRE (green) and ER-TR5 (a marker for medullary thymic epithelial cells; red) or for CD4 (green), CD8 (blue), and ER-TR5 (red). Representative results of 4 independent experiments are shown. Scale bars indicate 50 μm in panel A and 100 μm in panel B. (C) Two-color flow cytometry analysis of CD4 and CD8 in E18.5 thymocytes from indicated mice. Numbers indicate percentage of cells in quadrant. Representative results of 4 independent experiments are shown. (D) Means and standard errors (n = 3-4) of the numbers of thymocytes from indicated mice at E18.5 are shown. N.S. indicates not significant.