 Morgan’s fruit fly research proved that genes were on chromosomes. But by the 1950s it was still unclear what genes were made up of, what their molecular structure was, and how that structure produced the observable hereditary patterns found in nature. Biologists at the time debated whether the genetic material was deoxyribonucleic acid (DNA) or proteins; Oswald Avery, Colin MacLeod, and Maclyn McCarty had previously indicated in research on mice that it was DNA, but this conclusion was by no means universally accepted (Avery, MacLeod, and McCarty 1944). Alfred Hershey and Martha Chase at Cold Spring Harbor Laboratory ingeniously used viruses—made up of a protein body (or “coat”) and DNA inside—that infected bacteria to settle the matter (Hershey and Chase 1952). They first radioactively labeled the viruses’ protein coat and allowed them to infect the bacteria, and then they radioactively labeled the viruses’ DNA and allowed them to infect the bacteria. The question was: When the viruses replicated in the bacteria, would the radioactive marker show up in the bacteria infected by viruses with radioactively-labeled proteins or in the bacteria infected by viruses with radioactively-labeled DNA? It was the latter, and that convinced molecular biologists that deciphering the molecular structure of DNA was the next major challenge (entry:  molecular biology).