The 20S proteasome is composed of 28 subunits, arranged as a cylinder-shaped structure made of four stacked hetero-heptameric rings: α1-7, β1-7, β1-7, and α1-7. β1, β2, and β5 function the catalytic subunit for proteolysis. Murata and colleagues recently identified a new catalytic β subunit called β5t, which is only expressed in thymic cortical epithelial cells and dominantly incorporated into thymic 20S proteasomes. The integration of β5t into proteasomes reduces their chymotrypsin-like activity responsible for producing high affinity peptides for MHC class I. The depletion of β5t in mice resulted in defective development of CD8 single positive T cells in the thymus, indicating that β5t has a key role for generating the MHC class I-restricted CD8+ T cell repertoire during thymic selection. There results challenge the 15 years old theory of T-cell development, particularly positive selection in the thymus and the phenomenon of peripheral tolerance.