CD4(+) T cells are generally specialized to function as helper cells and CD8(+) 
T cells are generally specialized to function as cytotoxic effector cells. To 
explain how such concordance is achieved between co-receptor expression and 
immune function, we considered two possibilities. In one case, immature 
CD4(+)CD8(+) thymocyte precursors might first down-regulate expression of one 
co-receptor molecule, with the remaining co-receptor molecule subsequently 
activating the appropriate helper or cytotoxic functional program. 
Alternatively, we considered that the same intrathymic signals that selectively 
extinguished expression of one or the other co-receptor molecule might 
simultaneously initiate the appropriate helper or cytotoxic functional program. 
In the present study, we attempted to distinguish between these alternatives by 
examining thymocyte precursors of CD8(+) T cells for expression of Cathepsin C 
and Cathepsin W, molecules important for cytotoxic effector function. We report 
in developing thymocytes that Cathepsin C and Cathepsin W are expressed 
coordinately with extinction of CD4 co-receptor expression. We conclude that CD4 
extinction and initiation of the cytotoxic functional program occurs 
simultaneously during differentiation of CD8(+) T cells in the thymus.