And four times four gives us 16. And 16 is not enough to actually correspond to the 20 different amino acids that exist in nature. And that's exactly why we have to add one more nucleotide so that we have three consecutive nucleotides in our codon sequence, because four times four times four gives us 64 possibilities. And that is enough to basically describe the 20 different amino acids that exist in nature. Now, right away, you should notice that the genetic code contains 64 different codons in that particular genetic code. So 64 different variations of three letter sequences of nucleotides.