Internal Ribosome Entry Site (IRES) is a sequence of RNA that allows the ribosome to initiate protein synthesis without a 5' cap or initiation factors. IRES sequences have been identified in a variety of viral and cellular RNAs. Apart from IRES, there are several other synonyms for this term such as Internal Translation Initiation Site, Ribosome Shunting Site, Internal Entry Segment, and many more. While the basic functionality remains the same, each of the synonyms offers a slight variation in terms of the structural and functional characteristics of the IRES element. These synonyms are extensively used in molecular biology and genetic engineering studies.