The mouse cytomegalovirus (CMV), a b-herpesvirus, exploits its large(*230 kb) double-stranded DNA genome for both essential and non-essential functions. Among the non-essential functions are those that offer the virus a selective advantage in eluding both the innate and adaptive immune responses of the host. Several non-essential genes of MCMV are thought to encode MHC-I-like genes and to function as immunoevasins. To understand further the evolution and function of these viral MHC-I (MHC-Iv) molecules, X-ray structures of several of them have been determined, not only confirming the overall MHC-I-like structure, but also elucidating features unique to this family. Futureefforts promise to clarify the nature of the molecular ligands of these molecules, their evolution in the context of the adapting immune response of the murine host, and by analogy the evolution of the host response to human CMV as well.