Ribosome binding sites are a sequence of nucleotides upstream of the start (AUG) codon of an mRNA transcript. The ribosome recognizes and binds to a cap (a modified chemical structure) at the 5' end of the mRNA and moves along it until it finds the start codon or the first AUG codon. The Kozak sequence, which is a distinctive pattern of bases sometimes found around the AUG codon, helps to position the mRNA correctly.