The major pathway of gamma delta cell development is shown to be regul
ated by in-frame rearrangements at the T cell receptor (TCR) delta loc
us. Such ''delta selection'' occurs at or around the same point in thy
mocyte development as selection for in-frame rearrangements at the TCR
beta locus. However, there are at least two major differences with be
ta selection: first, delta selection commonly involves selection on th
e cognate TCR chain, gamma, suggesting that there is no ''preT gamma''
chain of major biological significance; second, most gamma delta-sele
cted thymocytes differentiate rather than proliferate. Nonetheless, so
me delta selection events seemingly facilitate thymocyte expansion, si
milar to alpha beta T cell development. In these cases, TCR gamma sele
ction is less obvious. Furthermore, the capacity of individual gamma c
hains to facilitate gamma delta selection is shown to vary with develo
pmental age. The results further clarify early T cell development at t
he beta selection/delta selection stage and place clear constraints on
models of cell fate determination.