American physician who was the first to announce that all cancer results from genetic change (or mutation) in cellular DNA. Earlier researchers had shown that genetic change could cause cancer, but had not stated that all cancer originated in this fashion. Horsfall's discovery opened the door to more intensive study of cancers and its genetic origin, leading in turn to a closer examination of the role that mutagens (such as certain chemicals, viruses, and radiation) can play in carcinogenesis.