The mechanism by which an initially uncommitted cell chooses between alternative 
fates is a central issue in developmental biology. In the mammalian thymus, CD4 
helper T cells and CD8 cytotoxic T cells arise from a common precursor that 
expresses both CD4 and CD8. The choice between the CD4 and CD8 lineage is linked 
to the specificity of the T-cell antigen receptor expressed by a thymocyte, but 
whether lineage commitment is stochastic or instructed has not been definitively 
resolved. We present evidence that expression of a constitutive CD8 transgene 
during thymic selection permits development of mature CD4 cells bearing the 
class I-restricted F5 T-cell antigen receptor. These results suggest that there 
is a stochastic component to the development of class I major histocompatibility 
complex-restricted T cells.