Recently it has been demonstrated that Tcell-mediated immunotherapy is anattractive treatment strategy for various diseases. For this therapeutic approach.the CD3 complex is one of the key targets to modulate T cell activation; however, in many instances we cannot directlyevaluate antibody candidates in mice because the monoclonal antibodies specific to human CD3 cannot cross -react to mouse endogenous CD3.In order to overcome the current model limitation, we have generated CD3e-extracellular domain KI mice for anti-human CD3e bispecific antibody in vivo efficacy evaluation.