In 1944, Avery and MacLeod, and McCarty purified the transforming principle of Fred Griffith's earlier experiment with pneumocossus bacteria. By a process of elimination, these scientists showed that DNA was the factor that caused genetic transformation.
A page from the May 15, 1943 letter from Oswald Avery to his brother Roy. In the letter Avery speculated on how transformation could happen. Avery never publicly connected genes with DNA and his transformation experiments.