„Substances capable of self-reproduction“ as cellular targets of chemical carcinogens. In the course of studies on chemical carcinogenesis, which included animal experiments with a carcinogenic azo dye and a mathematical analysis of the observed effects, Hermann Druckrey and Karl Kupfmuller showed in 1948 that carcinogens induce heritable changes by targeting cellular „substances capable of self-reproduction“. The authors did not discuss the chemical nature of these substances which remained unclear for a long time thereafter. It was not until 1964 that deoxyribonucleic acid (DNA) was recognized, by Peter Brookes and Philip Lawley, as a target for the genotoxic action of chemical carcinogens. In retrospect, the results of Druckrey and Kupfmuller gave an early, indirect and until now not considered hint to an important role of DNA (the „substance“) malfunction in the development of cancer, since DNA is the only molecule capable of self-reproduction.The results are now appreciated in light of the scientific knowledge available at the time.