Studies of herpes simplex virus type 1 (HSV-1) infection have shown that many known and unknown cellular molecules involved in viral proliferation are up-regulated following HSV-1 infection. In this study, using two-dimensional polyacrylamide gel electrophoresis, we found that the expression of the HSV-1 infection response repressive protein (HIRRP, GI 16552881) was up-regulated in human L02 cells infected with HSV-1. HIRRP, an unknown protein, was initially localized in the cytoplasm and then translocated into the nucleus of HSV-1-infected cells. Further analysis showed that HIRRP represses HSV-1 proliferation by inhibiting transcription of the viral genome by interacting with the cellular transcription factor, ATF5, via its N-terminal domain. ATF5 represses the transcription of many host genes but can also act as an activator of genes containing a specific motif. We found that ATF5 promotes the proliferation of HSV-1 via a potential mechanism by which ATF5 enhances the transcription of viral genes during the course of an HSV-1 infection; HIRRP then induces feedback repression of this transcription by interacting with ATF5.