In 1940, American Oswald Avery precipitates a pure sample of what he calls the transforming factor; he has isolated pure DNA for the first time.

Avery reported the results of more than 15 years of systematic attempts to identify the chemical nature of the substance that changes a heritable property of a bacterium. He demonstrated that DNA is the chemical equivalent of the then purely formal concept of genes. Considering the scientific developments during the remaining 20th century this was arguably the most important discovery in physiology or medicine of the century.