α:ß pathway
α chain begins re- arrangement when large double positive (DP) cells cease proliferation and become small DP cells
Begin to rearrange ß
• Cells fail to make successful β chainè die!
• ß chain with surrogate pTα is  expressed on surface (CD3:pre-T receptor complex)è cell proliferation, arrest further ß chain rearrangement, expression both CD4 and CD8
Majority of lymphocytes
DP cells have low levels of T cell receptorè positive and negative selection è Only ~2% DP become mature single positive (SP) CD4 or CD8 T cells and express high levels of TcR.