Intrathymic T-cell differentiation involves the generation, expansion
and selection of distinct T-lymphocyte subsets. While positive and neg
ative selection have been a focal point of T-cell development, these e
vents represent the final stages in a complicated sequence of differen
tiation steps. Here, Dale Godfrey and Albert Zlotnik summarize recent
advances in our understanding of early T-cell development and describe
five 'control points' that identify key events in this sequence.