As purification progressed, exposure of extracts to crystalline RNase and to proteinase preparations helped Avery's team determine that the biological activity of extracts was not dependent on RNA or protein. Crystalline DNase was not available until 1948, but biological activity was rapidly reduced by tissue extracts rich in DNase. McCarty's arrival at Rockefeller University was also marked by another milestone, namely, the development of a diphenylamine reagent assay to positively correlate DNA with biological activity. It gradually became evident that the active material in purified extracts had astonishingly high potency in micrograms of DNA that could consummate the pneumococcal transformation in vitro