Genetic code: the code that matches codons with specific amino acids. Since each codon is 3 nucleotides long (i.e., the %%genetic code%% is a triplet code) and there are 4 different RNA nucleotides (G, A, U, and C), the genetic code could in theory specify up to $4^3=64$ different amino acids. But since there are only 20 different amino acids used to synthesize proteins, the genetic code is redundant; that is, some amino acids can be specified by more than one codon. Also, the genetic code includes the start codon and the stop codons.