Current Evidence Indicates That >95% of the CD4⁺CD8⁺ Double-Positive Thymocytes $\alpha$

Essay

Current evidence indicates that >95% of the CD4⁺CD8⁺ double-positive thymocytes generated will die in the thymus, and will never develop into mature CD4 or CD8 T cells. While a small proportion of these double-positive thymocytes may fail to produce a functional $\alpha$ : $\beta$ T-cell receptor, the majority of them do express a T-cell receptor complex on their surface. For any given double-positive thymocyte undergoing cell death in the thymus, what are the two possible explanations for its failure to mature?