Serine/Tyrosine phosphatase which down-regulates cellular antiviral response by dephosphorylating activated host STAT1 and blocking interferon (IFN)-stimulated innate immune responses. Dephosphorylates the A17 protein. H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate Homodimer. Approximately 200 molecules of H1 are packaged within the virion and are essential for the viability of the virus. Expressed in the late phase of the viral replicative cycle. Belongs to the protein-tyrosine phosphatase family. Non-receptor class dual specificity subfamily.