Fig. 1 T cell development in the thymus
Upon TCR gene rearrangement, a broad TCR repertoire （nearly 1018） is created. Each DP cell expresses a unique TCR, and undergoes a selection process to determine its specificity to self-antigens presented by either MHC-I or MHC-II molecules that are mainly expressed on thymic epithelial cells. The reactivity of each TCR to self-MHCs determines the lineage fate.