Retains, but does not degrade MHC class I heterodimers in the endoplasmic reticulum during the immediate-early period of virus infection, thereby impairing their transport and maturation. Forms a complex with beta-2-microglobulin-associated class I heavy chains, which accumulate in the ER. In consequence, infected cells are masked for immune recognition by cytotoxic T-lymphocytes. Monomer. Expressed at immediate-early period of virus infection and at reduced levels at early-late times. The signal sequence is not cleaved. N-glycosylated; mostly exists in a high-mannose form. Belongs to the cytomegalovirus US2 family.