The majority of T cell receptor (TCR) complexes in mice and humans consist of a heterodimer of polymorphic TCRalpha and beta chains along with invariant CD3gamma, delta, epsilon, and zeta chains. CD3 chains are present as CD3gammaepsilon, deltaepsilon, and zetazeta dimers in the receptor complex and play critical roles in the antigen receptor assembly, transport to the cell surface, and the receptor-mediated signal transduction. That CD3 chains play critical roles in thymocyte development is apparent from the analyses of CD3 deficient mice. PreT cell receptor (preTCR)-mediated CD4(-)CD8(-) (double negative or DN) to CD4(+)CD8(+) (double positive or DP) transition is severely impaired in mice deficient in either CD3gamma, or epsilon, or zeta chain. In contrast, CD3delta deficiency impairs thymocyte maturation at the CD4(+)CD8(+) double positive (DP) stage suggesting that CD3delta is not required for the preTCR-mediated DN to DP transition. However, recent data suggest that a defect in human CD3delta results in impaired development at the DN stage indicating a role for hCD3delta in preTCR-mediated DN to DP transition. To determine if human CD3delta/epsilon (hCD3delta/epsilon) could mediate preTCR-mediated DN to DP transition, we employed a human CD3 transgene that encodes full length CD3delta and a truncated but functional form of CD3epsilon. Surprisingly, the transgene restored the defective preTCR function in not only CD3epsilon- but CD3gamma- and CD3gammadelta-deficient mice as well. A possible role for human CD3delta/epsilon heterodimer in the preTCR-mediated DN to DP transition is discussed.