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 o f 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' o f markers that shares the same timing o f expression across species barriers. It is proposed that T-cell markers that are members o f the invariant series are very important in deciding the fate of a developing thymocyte.