Internal ribosome entry site

An internal ribosome entry site, or IRES, is a DNA sequence that allows for translation initiation in the middle of the mRNA. 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.