It can be either the 5' cap of a messenger RNA in eukaryotes, or an internal ribosome entry site (IRES) in viruses, and is not to be confused with the Kozak consensus sequence (*In prokaryotes called the Shine-Dalgarno sequence), which is the initiation site for translation into amino acids. The ribosome usually starts searching for its site at one end of the mRNA but in some cases, particularly with viruses, there is an internal site called an (IRES).
