Participates in the inhibition of the host immune response. Redirects newly synthesized major histocompatibility complex (MHC) class I heavy chains via the SEC61 translocon to the cytosol where they undergo proteasome-dependent destruction. In consequence, infected cells are masked for immune recognition by cytotoxic T-lymphocytes. Interacts with host TRAM1. Expressed at early and late period of virus infection. Belongs to the cytomegalovirus US6 family.