A triplet of nucleotides within a molecule of messenger RNA that functions as a unit of genetic coding (the triplet code), usually by specifying a particular amino acid during the synthesis of proteins in a cell (see genetic code). A few codons specify instructions during this process (see start codon; stop codon). The term codon may also loosely refer to any of the corresponding nucleotide triplets of DNA that are transcribed into codons. See also reading frame. Compare anticodon.