Seems to function as a Vpr-like protein, since it mediates host cell cycle arrest in G2 phase. Cell cycle arrest creates a favorable environment for maximizing viral expression and production. Contains both nuclear import and nuclear export signals. Was first thought to be the equivalent of lentiviral Tat protein. PubMed:1323707 sequence does not seem to originate from isolate Petaluma as described, but rather from the strain San Diego.