The latency-associated transcript (LAT) is the major viral transcript detected by in situ hybridization of mouse and human sensory ganglia latently infected with herpes simplex virus type 1. The last 750 bases of LAT are complementary to infected-cell polypeptide 0, a herpes simplex virus type 1 immediate-early gene that encodes a transactivating protein that may facilitate re-activation of the virus from the latent state. Several laboratories have shown that LAT accumulates in the nucleus and is not polyadenylylated. Recently, we showed that the promoter for LAT lies 688 bases upstream from its 5' end. We report here that LAT is actually a uniquely stable intron. Furthermore, LAT effectively inhibits transactivation of gene expression by infected-cell polypeptide 0 in transient transfection assays.