host cells during latent infection; it is known as Latency Associated Transcript (LAT). A chromatin insulator region between promoters of the LAT and ICP0 genes of the HSV-1 genome may allow for the independent regulation of their expression. Silencing of ICP0 gene activity by ICP4 Although it is tempting to hypothesize that LAT is the repressor of the ICP0 gene, evidence supporting this hypothesis is lacking. Recent data suggest that ICP4 strongly suppresses the ICP0 gene, and ICP0 antagonizes ICP4. The balance between ICP0 and ICP4 dictates whether the ICP0 gene can be efficiently transcribed. Homologs across Herpes virus species The ICP0 gene and protein from HSV-1 have orthologs in related viruses from the herpes virus family. HSV-2 ICP0 is predicted to produce a polypeptide of 825 amino acids with a predicted molecular weight of 81986 Da, and 61.5% amino acid sequence similarity to HSV-1 ICP0. Simian varicella virus (SVV) is a varicellovirus that, like HSV-1 and HSV-2, belongs to the alphaherpesvirinae subfamily of herpes viruses. SVV expresses an HSV-1 LAT ortholog known as SVV LAT, and an HSV-1 ICP0 ortholog known as SVV ORF-61 (Open Reading Frame 61). Varicella Zoster Virus (VZV) is another varicellovirus in which a homolog of HSV-1 ICP0 gene has been identified; VSV ORF-61 is a partial homolog and a functional replacement for HSV-1 ICP0 gene. See also ICP-47 References Herpesviridae Proteins