Evidence from a number of virus-cell systems, discussed above and in other papers in this section, shows that the resistance to virus infection induced by interferon is characterized by inhibition of both primary transcription and translation. This may be because interferon induces two (or more) distinct "antiviral proteins" which inhibit, respectively, the transcription and the translation of virus genetic information. Although both may be present in interferon treated cells, the one which is manifest in any given experimental system may depend on whether transcription or translation is the primary event in the replication of the particular virus, and upon the accessibility of the viral nucleic acid. Alternatively, inhibition of transcription and translation may be mediated by a single antiviral protein which is able to distinguish viral from cellular nucleotide sequences in both DNA and RNA. The interferon-induced ribosome-associated inhibitors of viral mRNA translation (see related papers in this section) are prime candidates for such an antiviral protein.