Editing T cell (section)

====TCR β-chain selection====
At the DN2 stage (CD44<sup>+</sup>CD25<sup>+</sup>), cells upregulate the recombination genes RAG1 and RAG2 and re-arrange the [[Thyroid hormone receptor beta|TCRβ]] locus, combining [[V(D)J recombination|V-D-J recombination]] and constant region genes in an attempt to create a functional TCRβ chain. As the developing thymocyte progresses through to the DN3 stage (CD44<sup>−</sup>CD25<sup>+</sup>), the thymocyte expresses an invariant α-chain called pre-Tα alongside the TCRβ gene. If the rearranged β-chain successfully pairs with the invariant α-chain, signals are produced which cease rearrangement of the β-chain (and silence the alternate allele).<ref>{{Cite book|last=Murphy|first=Kenneth|title=Janeway's Immunobiology|publisher=Garland Science|year=2011|isbn=9780815342434|edition=8th|pages=301–305}}</ref> Although these signals require the pre-TCR at the cell surface, they are independent of ligand binding to the pre-TCR. If the chains successfully pair a pre-TCR forms, and the cell downregulates CD25 and is termed a DN4 cell (CD25<sup>−</sup>CD44<sup>−</sup>). These cells then undergo a round of proliferation, and begin to re-arrange the TCRα locus during the ''double-positive'' stage.