Codon (triplet, encoding tri-nucleotide) is a sequence of three neighbouring nucleotides in a DNA or a RNA, which encodes either a certain amino acid, or beginning (in this case it is called a start-codon, or, an initiation codon, or, an initiator), or stoppage (stop-codon, nonsense codon) of protein translation. Codon is a descrete unit of a genetic code. There are 43=64 various possible combinations of nucleotides in triplets. 61 of them encode 20 amino acids, and three of them are stop-codons.