The preTCR is associated with signal-transducing CD3␥, ␦, , and polypeptides. It is generally agreed that CD3 chains play redundant roles in the receptor-mediated signal transduction. In the present study, we show that the intracytoplasmic (IC) domain of CD3 is essential for early thymocyte maturation. We demonstrate that the IC domain-deleted CD3 fails to restore the double negative (DN) to double positive (DP) thymocyte development in CD3-deficient mice. Additional experiments show that the membrane proximal basic amino acid rich sequence in the IC domain of CD3 is sufficient for the DN to DP differentiation, whereas the proline rich sequence is required for efficient proliferation. This is probably due to impaired ligand independent recruitment of Nck to the proline rich sequence motif of CD3 within the context of the preTCR. The data presented in this study elucidates mechanistic basis for the preTCR-induced proliferation of the DN thymocytes and have identified distinct roles for individual motifs of CD3 in the preTCR-mediated differentiation and proliferation. These data provide the first genetic and phenotypic evidence for requirement of the IC domain of a CD3 chain in thymocyte development. ϩ (double positive or DP) and thymic selection of the DP thymocytes results in the generation of CD4ϩ (single positive or SP). The preTCR, which consists of a pT␣/TCR heterodimer and signal transducing invariant CD3␥-, ␦-, -, and -chains, mediates the DN to DP transition (1-3). The preTCR is expressed at a low level on the DN thymocytes and is suggested to be due to the endoplasmic reticulum (ER) retention and constitutive internalization of the surface receptor (4 -7). It is generally agreed that the preTCR functions in a ligand-independent manner. The oligomerization of preTCR via the pT␣-chain has been proposed as the mechanism for the preTCR-mediated cell autonomous signal transduction (8). The surface expression of the preTCR initiates multiple intracellular signaling pathways, culminating in the DN to DP differentiation and clonal expansion, resulting in a several-fold increase in thymic cellularity (3). Several studies suggest that the preTCR-mediated differentiation and clonal expansion of the DN thymocytes are two distinct processes (reviewed in Ref. 9). Thus, transgenic expression of dominant-negative fas-associated death domain, or abrogation of either early growth response gene 3 or c-Myc transcription factor impairs proliferation, but not differentiation, of the DN thymocytes (10 -12). However, it is not clear whether the preTCR is capable of directly inducing multiple pathways each resulting in distinct biological processes.CD3 chains play multiple roles in the preTCR-mediated DN to DP thymocyte differentiation. These include ordered intracellular assembly and transport to the cell surface, receptor-induced differential signal transduction, and receptor internalization (ligand-induced and independent). CD3 chain specific transcripts are expressed as early as DN1 stage and almost all the DN3 thymocytes e...