who became professor of biochemistry at Columbia University (1952–74). Stimulated by Oswald Avery’s identification of DNA as the basis of heredity in pneumococcus bacteria, he discovered that the constitution of DNA is consistent within a species but that there are as many types of DNA as there are species. However, his discovery that the number of purine bases is always equal to the number of pyrimidine bases, and that a similar relationship exists between adenine and thymine bases and between cytosine and guanine bases—a principle known as Chargaff’s rule—provided Watson and Crick with an important clue in their elucidation of the chemical basis of heredity.