The function of genes, known since the work of Beadle and Tatum in 1945 and Linus Pauling in 1949, is to code for the structure of proteins (more accurately, polypeptide chains). The disciplines of biochemistry and genetics are united in the DNA nucleotide sequence of a gene coding for the amino acid sequence of a polypeptide chain. The latter sequence determines not only how a protein folds into three dimensions but also the specific enzymatic reaction(s) or other chemical role(s) of the protein in the organism. A considerable body of effort over the past 50 years consisted of finding which proteins were coded by which genes. Starting with an isolated protein, the amino acid sequence was used to predict the nucleotide coding sequence. Eventually, an oligonucleotide with this sequence could be made and used by hybridization to isolate or locate the chromosomal gene. Starting with a mutant gene in an organism, biochemical assays are used to determine which protein is aberrant, or mapping studies are used to determine