Three codons do not encode any amino acid but instead serve to mark the end of the protein. However, in some very specific proteins two of these codons encode the two very rare amino acids (selenocysteine and pyrrolysine). They are not present in majority of the proteins and require additional regulatory sequences to be included. | Three codons do not encode any amino acid but instead serve to mark the end of the protein. However, in some very specific proteins two of these codons encode the two very rare amino acids (selenocysteine and pyrrolysine). They are not present in majority of the proteins and require additional regulatory sequences to be included. |