If three nitrogen bases form a code, the arrangement or assembly of four nitrogen bases will be 4×4×4=64. That is, the triplet codon number is 64. 64 codes can code for 20 amino acids. The number of functional codons is 61 and termination codons are 3. In 1964, Nirenberg and Matthaei discovered 64 triplet codes for 20 amino acids.