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ABSTRACT: Mouse cytomegalovirus (MCMV), a β-herpesvirus that establishes latent and persistent infections in mice, is a valuable model for studying complex virus-host interactions. MCMV encodes the m145 family of putative immunoevasins with predicted MHC-I structure. Functions attributed to some family members include downregulation of host MHC-I (m152) and NKG2D ligands (m145, m152, m155) and interaction with inhibitory or activating NK receptors (m157). We present the cellular, biochemical and structural characterization of m153, which is a heavily glycosylated homodimer, that does not require β2m or peptide, and is expressed at the surface of MCMV-infected cells. Its 2.4 Å crystal structure confirms that this compact molecule preserves an MHC-I-like fold and reveals a novel mode of dimerization, confirmed by site-directed mutagenesis, and a distinctive disulfide-stabilized extended amino terminus. The structure provides a useful framework for comparative analysis of the divergent members of the m145 family.