Ralph Budd and Philip Mixter present a hypothesis suggesting that CD4-CD8-TCR alpha beta+ cells arising in the normal thymus result from a high-avidity T-cell receptor (TCR) signal bordering on negative selection. In normal mice, this subset is likely to be gradually deleted but, in the absence of Fas, these cells persist in lpr mice. The model they describe makes several predictions regarding the nature of this unusual T-cell subset.