Schematic of the T cell receptor/CD3 (TCR/CD3) complex on the surface of a normal CD4^–CD8^– double-negative thymocyte. All but one of the normal chains of the CD3 complex, including the β chain, are present, but instead of the CD3α chain, the pre–T cell receptor α gene is expressed at this stage. It is postulated, based on the CD3ε murine knockout studies, that the block in T cell development caused by mutations in CD3ε occurs at or just after this stage. In their study in this issue, de Saint Basile and colleagues (13) report histologic data which suggests that, in the case of mutations in the CD3δ gene, the block in T cell maturation occurs at the next stage, i.e., at the intermediate single-positive stage.