Sixteen years later, in 1944, Oswald Avery, Collin Macleod and Maclyn McCarty attempted a more definitive experiments. They purified DNA, RNA, proteins and other materials from heat killed S-type and mixed with R-type to see which ones could transform living R-type to S-type. Only those mixed with DNA were transformed mixed with Deoxyribonuclease ( the enzyme that will digest and destroy DNA ) there was no transformation. For their experiments they used a test tube assay instead of mice.
These experiments prove that the transforming principle is DNA but all biologists were not convinced.
Finally, Alfred Hershey and Martha Chase ( 1952 ) proved that DNA is the genetic material and not protein using bacteriophages.