Griffith thought some "transforming principle" from the heat-killed strain was responsible for making the harmless strain virulent. In 1944 this transforming principle was identified as being genetic by Oswald Avery, Colin MacLeod, and Maclyn McCarty. They isolated DNA from a virulent strain of "S. pneumoniae" and using just this DNA were able to make a harmless strain virulent. They called this uptake and incorporation of DNA by bacteria 'transformation'. See Avery–MacLeod–McCarty experiment.