Internal Ribosome Entry Sites (IRES)

The vast majority of eukaryotic mRNAs are translated through the ribosome scanning mechanism. An alternative mechanism is internal ribosome entry. The mRNAs lack a 5’ cap, and translation is initiated at internal ribosome entry sites (IRES). IRESs have a complicated tertiary structure and bind 40S subunits in close proximity to an AUG codon.