T-cell development in the fetus and the invariant series hypothesis.

Comparison of the timing of appearance of certain T-cell markers in the intrathymic development of T cells during gestation reveals a common sequence of expression in several species. Here Richard Aspinall and colleagues put forward a hypothesis concerning this 'invariant series' of markers that shares the same timing of expression across species barriers. It is proposed that T-cell markers that are members of the invariant series are very important in deciding the fate of a developing thymocyte.