The phenotype of thymocytes undergoing positive and negative selection in the HY^cd4 model. (A) CD4 by CD8 profiles of T3.70⁺ thymocytes from female and male mice. (B) The expression of CD69 and PD-1 on thymocytes of the indicated genotype. The percentage of PD-1⁺ cells in the HY^cd4 male is indicated. (C) The activation of Caspase 3 in male mice compared with female mice. The fold change in male over female is indicated. Data represent the mean from 16 individuals from 12 different experiments ± SD. P < 0.0001.