A and G (adenine and guanine), and two pyrimidines, C and T (cytosine and thymine); in mRNA, U (uracil) replaces T. The words of the alphabet comprise 3 letters, thus there are 43=64 permutations or words which are called codons. 61 of the 64 codons code for 1 of 20 amino acids. Since more than one codon codes for a particular amino acid the code is said to be redundant. Four of the 64 codons punctuate the message; one, AUG, is the start signal and three, UAG, UAA and UGA, are stop signals.