In the thymus, immature T cells differentiate from common precursors to become T cells expressing either the αβ or γδ T cell receptor (TCR) complex. The CD3ε subunit of the TCR complex is thought to transduce ligand-induced conformational changes in the TCR by recruiting the cytosolic adaptor protein Nck. To investigate the role of conformational changes in the TCR in T cell development, we generated mice with a germline mutation (C80G) in the extracellular domain of CD3ε, which prevents the outside-in transmission of conformational changes in the TCR. The development of αβ T cells in the C80G mice was blocked at an early stage that depends on signaling by a precursor form of the TCR. In contrast, the C80G mutation did not impair the development of some subsets of γδ T cells, including Vγ1.1⁺ cells; however, development of other γδ T cell subsets was blocked. A similar phenotype was observed in mice with a mutation in the cytoplasmic proline-rich sequence (PRS) of CD3ε, the binding site for Nck. In a genetic complementation test, the PRS CD3ε mutant failed to rescue the wild-type phenotype when expressed in heterozygosity with the C80G mutant. These data suggest that Nck may function as an effector of TCR conformational changes during T cell development. Additional experiments showed differential effects of the C80G mutation on the activation of TCR-dependent signaling pathways, which suggests that there are pathways that are either dependent on or independent of the transmission of conformational change in the receptor.