RADIATION STANDARDS, INCLUDING FALLOUT 335 the DNA molecules of a cell, it can be demonstrated to have a comparable action in all cases. It fractures and disrupts the chromosomes which contain the DNA; it blasts the genes, usually with destructive effects; and it causes chromosomes to stick together and fail to enter the daughter cells properly, at the time when the parent cell is dividing. We may expectto find differences, mainly quantitative ones, in the responses to radiation of different species. For that matter, it is now quite clear that the DNA and the chromosomes ofthe fruit fly’s spermatozoa are not at the same level of susceptibility as those in the oocytes of the female fruit fly during her maturity. It is also true that the DNA and the chromosomes in the immature germ cells are far less susceptible to radiation than those in the mature germ cells. So it is not surprisingto find that the reproductive cells of a mouse are more susceptible than those of a fruit fly, as indeed they are. What we need is more information of this sort from a variety of animal species; but the longer the animals live, and the fewer offspring each female can produce, the more laborious and expensive the experiments must be. The Atomic Energy Commission, Division of Biology and Medicine, is now supporting an experimental genetic study of mutation in irradiated pigs, but as may well be imagined, progress is slow and the expenseis high. Wemaybeable to get at direct evidence of production of mutations in humansby radiation and of the quantitative level at which mutations are produced in relation to dosage. By growing human cells in artificial culture media and exposing them to ionizing radiation, a number of workers—Puck and Benderin this country, and Dubinin and his group in Russia—have demonstrated that the chromosomes are fractured and rejoined in various ways, and the numberof chromosome breaks is linearly proportional to the dose. This has been demonstrated over a range from 10 roentgens up to several hundred roentgens. Moreover, the number of breaks produced by a given dose in humancells may be compared with the number produced by the same dose in comparable tissue cells of a monkey, mouse, or hamster. Benderhas recently reported the results of irradiating with X-rays white blood cells freshly drawn from the human body, and then culturing them long enough to determine the frequency of chromosome breaks in these cells. A linear proportionality between frequency of breaks and radiation dose is demonstrated. In my ownlaboratory, J. G. Brewen has used the corneal epithelium of the hamster for similar experiments, irradiating the tissue in its natural location in the eye, and again finds a range of doses from 25 to ie roentgens yields linear proportionality of chromosome breaks to dose. Since the 1960 report of the National Academy of Sciences Commit- tee on the Genetic Effects of Atomic Radiation, a quite new aspect of the problem has come into prominence. In 1959 it was discovered in France and Great Britain that most of mongoloid idiocy are attributable to the presence in the cells of an extra chromosome, & very small one, No. 21 amongthe 23 pairs of human chromosomes. Shortly afterward, two forms of sexual maldevelopment accompanied bysterility were discovered to arise, the one from the presence of an extra sex chromosome, the other from the lack of a sex chromosome normally SEigEatnC haggistong nage aeIECEEERERN