TdT: Throughout the developmental period
Cytoplasmic protein tyrosine kinase Lck (if deficient: T cell is arrested before double positive stage and no α chain) ZAP70: promote single positive thymocytes from double positive ones
Transcription factors: guide development and often have roles in mature T cells Inappropriate expression of Notch-lymphomas;
RAG1 and 2 are repressed once pre-T cell receptor is presentè proliferate è α rearrangement begins. This ensures β is successfully rearranged and gives rise many double positive  cells with different α
The temporal pattern of relevant cellular proteins in T-cell development