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. Author manuscript; available in PMC: 2011 Apr 1.
Published in final edited form as: Curr Opin Immunol. 2010 Jan 13;22(2):185–192. doi: 10.1016/j.coi.2009.12.006

Figure 2. Evolution of TCRs and lineages.

Figure 2

The four TCR chains are present in all vertebrates that undergo Rag-dependent rearrangement. It is not clear when the molecular programs corresponding to αβ and γδ lineages were always present, but the DP stage can be found in Xenopus, suggesting that certain features of the αβ lineage existed before the split between amphibians and reptiles. pTα homologs, however, can be found only in mammals, suggesting that αβ lineage differentiation initially did not rely on pre-TCR signaling. (mya – million years ago)