Objectives: To avoid recognition by CD8+ T cells, mouse cytomegalovirus (MCMV) downregulates MHC I molecules from the surface of infected cells. However, complete downregulation of MHC I makes infected cells susceptible to NK cells lysis. Therefore, MCMV encodes for m04 protein that binds some MHC I molecules in the infected cells, escorts them to the cell surface where they engage inhibitory Ly49 receptors and inhibit NK cell response strongly than uninfected cells. Although m04 protein is abundant inside the cell, only small proportion goes to the cell surface and binds to MHC I with surprisingly low affinity. Our preliminary results indicate the role of MAT transcript in the mentioned interaction. Therefore, the aim of this study is to clarify the function of MAT transcript in the regulation of the immune response, with an emphasis on Ly49 receptors and NK cells. Materials and Methods: To clarify the role of Ly49 receptors in the recognition of viral-infected cells, we used reporter cells expressing Ly49 receptor. To determine the role of MATp1 protein in maturation and expression of MHC I molecules, we have metabolically labeled MHC I and immunoprecipitated them. The mechanism of interaction between MATp1, m04 and MHC I was determined using co-immunoprecipitations and proximity ligation assay (PLA). Newly generated monoclonal antibody α-m04 was used in these methods. We used the plaque assay in order to determine the role of MATp1 on the control of MCMV infection in vivo. We also performed functional tests and phenotype analysis of NK cells and CD8+ T lymphocytes by flow cytometry. Results: MATp1 interacts with MHC I (H-2Dd) and m04, thereby enabling m04/H-2Dd complex to reach the cell surface. There, MATp1/m04-modified altered-self MHC I molecules bind inhibitory Ly49A receptors more strongly than MHC I molecules alone. This results in NK cell inhibition and inability of viral control in early days post infection. To counter this, host developed activating Ly49D2, Ly49L and Ly49P receptors specific for MATp1/m04-modified altered-self MHC I molecules. However, MCMV mutated MATp1 and m04 therefore escaping the recognition by activating Ly49 receptors and consequently created new variants of MCMV. MATp1 also negatively affects the virus-specific CD8+ T cell response during infection. Conclusion: Results of this research answer a long-standing conundrum of how MCMV avoids recognition by NK cells in most mouse strains despite reduced surface levels of MHC I, reveals a fundamental new mechanism of viral immune evasion involving the interaction of the virally-altered MHC I molecules and inhibitory Ly49 receptors and demonstrate how this interaction forced the evolution of virus-specific activating MHC I-restricted Ly49 receptors.