Fred Griffith, a British medical researcher, had injected a nonvirulent strain of pneumococci (R-pneumococci) in mice along with dead cells of a virulent form of pneumococci (S-pneumococci). The mice died and live S-pneumococci were found in their lungs. Avery set out to answer the question: How did this occur?

Researchers in Avery's laboratory eventually duplicated Griffith's results in a test tube, using bacteria instead of mice. They found that replacing the dead S-bacteria with a crude cell-free extract of the same bacteria transformed the nonvirulent bacteria into the virulent strain. The finding had far-reaching implications.

In 1944, Avery and two senior associates, Colin MacLeod and Maclyn McCarty, published their conclusion: DNA, and DNA alone, was the material with genetic properties. This finding was a direct challenge to the then-current dogma that only proteins - more complex and more intricately folded molecules - existed in the multitude of forms needed to store the genetic blueprint for an entire organism.