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What is an IRES?
An internal ribosome entry site, abbreviated IRES, is a nucleotide sequence that allows for translation initiation in the middle of a messenger RNA (mRNA) sequence as part of the greater process of protein synthesis. Usually, in eukaryotes, translation can only be initiated at the 5' end of the mRNA molecule, since 5' cap recognition is required for the assembly of the initiation complex. The IRES mimics the 5' cap structure, and is recognized by the 43S pre-initiation complex. IRES sequences are very rare, and are mainly observed in viral mRNAs.