A ribosomal binding site (RBS) is a sequence on mRNA that is bound by the ribosome when initiating protein translation.
It can be either the 5' cap of a messenger RNA in eukaryotes, a region 6-7 nucleotides upstream of the start codon AUG in prokaryotes (called the Shine-Dalgarno sequence), or an internal ribosome entry site (IRES) in viruses. The sequence is complementary to the 3' end of the rRNA. The ribosome searches for this site and binds to it through base-pairing of nucleotides. Then, the ribosome begins the translation process and recruits initiation factors. After finding the ribosome binding site in eukaryotes, the ribosome recognizes the Kozak consensus sequence and begins translation at the +1 AUG codon.