The sequence of the bases of an mRNA (A, U, G, C) directly spells out the sequence of amino acids in the protein.

Every three bases in the mRNA sequence codes for a single amino acid and is called a "codon." There are 64 different ways to arrange the four bases in mRNA, e.g., UCA, UCG, AAA, and AGC, and yet there are only 20 different amino acids. This means that one amino acid can have more than one codon. For example, valine has four codons (GUU, GUC, GUA, and GUG), while histidine only has two (CAU and CAC).

The mRNA code also has one "start" and three "stop" signals. The codon AUG codes for an amino acid called methionine that always signals the start of protein building. Three other codons--UAG, UGA, and UAA--do not code for any amino acid. Instead, they always signal the mRNA to stop protein building.