HYα⁺ cells in male HY-STOP mice are negatively selected. (A) Expression of CD4 and CD8 co-receptors by male and female HYα⁺ thymocytes. Representative results of at least five independent experiments are shown. Note co-receptor dulling and lack of CD4⁻CD8⁺ cells among male clonotype-positive cells. (B) Expression of negative selection–associated markers PD-1 and Helios by total HYα⁺ DP thymocytes from male and female HY-STOP mice and by DP^dull cells from male mice. Representative results of three independent experiments are shown. (C) Detection of cleaved caspase-3 in male and female HYα⁺ thymocytes from HY-I (top) and HY-STOP (bottom) mice. Representative results of two independent experiments are shown.