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; approximately 50 kd) was a protein recognized by B291. It was detected in 12-O-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 (approximately 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.