Objective:To identify and characterize an open reading frame (ORF) in association with its mRNA and a coding protein recognized by anti-human herpesvirus 8 (HHV-8) monoclonal antibody B291.Study Design/Methods:The antigen detected by B291 was characterized by immunofluorescence method, Western blot analysis, laser confocal microscopy, and immunohistology. cDNA of the protein was identified by immunoscreening the HHV-8 cDNA library with B291 and sequenced. Structure and kinetics of the mRNA expression was investigated by 5′,3′ rapid amplification of cDNA ends (RACE) and Northern blot analysis.Results:Viral interferon regulatory factor (vIRF; ~50 kd) was a protein recognized by B291. It was detected in 12-0-tetradecanoyl phorbol-13 acetate (TPA)-induced, but not uninduced, HHV-8-infected cell lines (BCBL-1 and BCP-1) and the tumor cells obtained from a SCID mouse injected with uninduced BCBL-1. The expression of vIRF was predominantly in the nucleus and was partially diminished by phosphonoformic acid (PFA) treatment, whereas the mRNA (~2.4 kb) was accumulated.Conclusions:vIRF is an early protein expressed in the nucleus and may be involved in Kaposi's sarcoma tumor formation.Journal of Human Virology 1999;2:63-71 © Lippincott Williams & Wilkins, Inc.