A viral FLIP (FLICE/caspase-8-Inhibitory Protein), equine herpesvirus type 2 E8 protein, has been shown to inhibit Death receptor-induced apoptosis by suppressing the activation of FLICE/caspase-8. We generated transgenic mice specifically expressing E8 in thymocytes under the control of lck-proximal promoter. Although E8-expressing thymocytes were resistant to Fas-mediated apoptosis, the total number of thymocytes in 4-8-week-old E8 transgenic mice was more than 3-fold less than that in control littermates. This reduction was also observed in E8 transgenic mice with a Fas-/- background suggesting the reduction to be independent of Fas. The thymocytes of the transgenic mice, however, could similarly respond to CD3-mediated stimulation, indicating that the reduction of thymocyte numbers might be independent of T cell receptor complex-mediated stimulation. Thus, the Death receptor-mediated signaling pathway is too complex to be regarded as only an executor for apoptosis.