Plays a role in the inhibition of host immune response. Binds specifically to transporters associated with antigen processing (TAP), thereby blocking peptide-binding and translocation by TAP as well as subsequent loading of peptides onto MHC class I molecules. Empty MHC I molecules are retained in the endoplasmic reticulum and ultimately directed to proteasomal degradation. In consequence, infected cells are masked for immune recognition by cytotoxic T-lymphocytes. Interacts with host TAP1 and TAP2; these interactions inhibit the loading of peptides onto MHC class I molecules. The N-terminal active domain blocks peptide binding to and peptide transport by TAP. Belongs to the herpesviridae US12 family.