In the early 1940s Oswald T. Avery and Maclyn McCarty, a colleague at the Rockefeller Institute Hospital, began concentrating on the problem of pneumococcal transformation. Avery's work focused first on purifying the transforming substance. Using refined versions of Colin M. MacLeod's preparation techniques, Avery and McCarty isolated biologically active "transforming principle" from samples of pneumococci. Then attention turned to its chemical analysis. Proteases -- enzymes that deactivate proteins -- and lipases -- enzymes which destroy lipids -- were found not to inactivate the transforming principle. Avery could thus conclude that the substance was essentially protein and lipid free. Avery had found that the substance was rich in nucleic acids, but ribonuclease, an enzyme that destroys ribonucleic acid (RNA), did not inactivate the substance either. It was definitely not a carbohydrate like the polysaccharide capsular material, as carbohydrates are not precipitated by alcohol, as was the "transforming principle." Alcohol was, however, a well-known precipitant for deoxyribonucleic acid (DNA). Further, the transforming substance had a high molecular weight, as did DNA, and gave a strong reaction to the Dische test for DNA. The transforming substance, which produced permanent, heritable change in an organism, was deoxyribonucleic acid.
Avery's brother Roy (a bacteriologist at the Vanderbilt School of Medicine) was one of the first persons outside of Avery's laboratory to be informed of Avery's findings. In May, 1943, Avery wrote his brother to tell him of the fruits of the past two years of research. "Sounds like a virus," Avery wrote, "may be a gene."
Avery, MacLeod, and McCarty published their discovery that the transforming principle was DNA in 1944 in the Journal of Experimental Medicine. Their conclusions in this paper were cautious, and they presented several interpretations of their results. The phenomenon of transformation, Avery wrote, "has been interpreted from a genetic point of view. The inducing substance has been likened to a gene, and the capsular antigen which is produced in response to it has been regarded as a gene product." Yet they also gave another interpretation, that there might be an "analogy between the activity of the transforming agent and that of a virus." They concluded: "Assuming that the sodium desoxyribonucleate and the active principle are one and the same substance, then the transformation described represents a change that is chemically induced and specifically directed by a known chemical compound. If the results of the present study on the chemical nature of the transforming principle are confirmed, then nucleic acids must be regarded as possessing biological specificity...."