HY^cd4 mixed BM chimeras as a model system to study clonal deletion in vivo. (A) HY^cd4 TCRα^o female BM was mixed with congenic B6.PL female and male BM and transferred into congenic female and male hosts. Chimeras were harvested at 6 wk and thymi were analyzed by flow cytometry for expression of Thy1.2, T3.70, CD4, CD8, and active Caspase 3. HY^cd4 donor-derived cells were identified by Thy1.2 where indicated. Numbers represent frequency of parent gate. (B) The activation of Caspase 3 in male chimeras compared with female chimeras. The fold change in male over female is indicated. Data represent the mean from 15–17 individuals from six different experiments ± SD. P < 0.0001.