A similar genetic code is used by most organisms on Earth, but different organisms have different preferences for the codons they use to encode specific amino acids. This is possible because there are 4 bases (A, T, C, and G) and 3 positions in each codon. There are therefore 64 possible codons but only 20 amino acids and 3 stop codons to encode leaving 41 codons unaccounted for. The result is redundancy; multiple codons encode single amino acids. Evolutionary constraints have molded which codons are used preferentially in which organisms - organisms have codon usage bias.