Recent investigations reveal that CPXV203 binds a diverse array of classical and non-classical MHC class I proteins. CPXV203 works synergistically with the KDEL receptor to block MHC class I surface expression. CPXV203 binds MHC class I with dramatically increased affinity at the lower pH of the Golgi relative to that of the ER. Thus CPXV203 disrupts trafficking of fully assembled MHC class to the cell surface. By preventing antigen display on MHC class I, CPXV203 acts to disable adaptive immune system surveillance of infected cells.