Positive selection of T cells in the thymus is induced by low-affinity TCR recognition of self-peptide-MHC complexes expressed by cortical thymic epithelial cells (cTECs). cTECs express a specialized type of proteasomes, the thymoproteasome, which generates a unique spectrum of MHC class I-associated peptides and plays a critical role in thymic positive selection of CD8 T cells. However, it remains unclear how the thymoproteasome contributes to the thymic positive selection. More than 30 years ago, the "peptidic self" hypothesis proposed that TCRs recognize MHC-presented peptides only, without interacting with MHC molecules, which turned out to be incorrect. Interestingly, however, by implying that a set of MHC-associated peptides forms immunological self, this hypothesis also predicted that positive selection in the thymus is the primary immune response to "foreign epitope" peptides during T cell development. The thymoproteasome-dependent unique self-peptides may create those foreign epitope peptides displayed in the thymus for positive selection of T cells.