IRES stands for "internal ribosome entry site," which is a segment of RNA that can initiate protein synthesis even without a 5' cap. Some synonyms for IRES include internal translation initiation site, ribosome shunt, and alternative translation initiation site. These terms refer to the same function of allowing translation to begin within the middle of an mRNA instead of at the 5' end. IRES elements are found in the genomes of viruses, but also in the mRNAs of some vertebrates and insects. Understanding the mechanisms of IRES-mediated translation can provide insight into gene expression regulation and offer therapeutic targets for virus infections and genetic diseases.