Genetic code
The instructions in a gene that tell the cell how to make a specific protein.
A, T, G, and C are the 'letters' of the DNA code.
They stand for the chemicals adenine, thymine, guanine, and cytosine, respectively, that make up the nucleotide bases of DNA.
Each gene's code combines the four chemicals in various ways to spell out 3-letter 'words' that specify which amino acid is needed at every step in making a protein.
The discovery of the genetic code clearly ranks as one of the premiere events in what has been called the Golden Age of Biology and Medicine.