Artificial transmutation of the gene.

Muller showed that radiation causes mutations that are passed on from one generation to the next. This was the first suggestion that inherited traits might be altered or controlled, and it created a sensation: “Man’s most precious substance, the hereditary material which he could pass on to his offspring, was now potentially in his control. X rays could speed up evolution,’ if not in practice at least in the headlines. Like the discoveries of Einstein and Rutherford, Muller’s tampering with a fundamental aspect of nature provoked the public awe.“(Carlson). See also Muller's follow-up paper, "The production of mutations in x-rays," Proc. Nat. Acad. Sci. 14 (1928) 714-26. 

In 1946 Muller was awarded the Nobel Prize in Physiology or Medicine "for the discovery of the production of mutations by means of X-ray irradiation."