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Introduction of bispecific adapter cassettes into the coronaviral genome. Schematic representation of the MHV genome and of the bispecific expression cassettes introduced herein. The plus-strand RNA genome contains, from 5′ to 3′, the polymerase precursor gene (ORF1ab), the accessory genes 2a and 2b/HE, the S gene, the nonessential genes 4ab and 5a, and the genes encoding the virion proteins E, M, and N. In the recombinant viruses, the gene cluster 2a + 2b/HE was replaced with an expression cassette downstream of the translation regulation sequence for protein 2a. Recombinant MHV viruses were generated by targeted recombination . Indicated is whether the particular recombinant virus could be isolated, as confirmed by RT-PCR on virus RNA. In addition, the ability of such viruses to infect murine and human cells is depicted (Verheije and Rottier, unpublished data). Abbreviations of adapters as specified in the text: “na”: not applicable.