 The association of exposure to radiation and cancer had been observed as early as 1902, six years after the discovery of X-ray by Wilhelm Röntgen and radioactivity by Henri Becquerel.Georgii Nadson and German Filippov were the first who created fungi mutants under ionizing radiation in 1925.The mutagenic property of mutagens was first demonstrated in 1927, when Hermann Muller discovered that x-rays can cause genetic mutations in fruit flies, producing phenotypic mutants as well as observable changes to the chromosomes, visible due to the presence of enlarged "polytene" chromosomes in fruit fly salivary glands.