MUTAGENS OF POTENTIAL SIGNIFICANCE 175 



Glass: In our tissue culture studies, the frequency of spontaneous 

 breakage, from Bender's data (7,8), ran around 1 per cent of untreated 

 cells with chi^omosome breaks, so that you could say that the ordinary 

 methods of culturing certainly are producing a fairly high percent- 

 age of chromosome disruption, unless you want to suppose that this 

 goes on in vivo, too. I don't really think there is any warrant for that. 



Atwood: There is good evidence against it, in fact. 



Glass: Yes. The agents in plants mostly have not been tried as yet 

 in tissue culture. We are just beginning this kind of work. 



Auerbach: What strikes me is that Drosophila chromosomes, at 

 least those in the germ cells, require high doses for breakage. To get 

 a moderate yield of translocations one uses at least 1000 r. For plants, 

 this would be a very high dose. I think, though, that in tissue cultures, 

 animal chromosomes can also be broken by low doses. Did not Puck 

 find an effect with only a few rf 



Glass: Yes. Bender finds that a dose of about 200 r would produce 

 at least one chromosome break in every cell. 



Auerbach: Drosophila sperm chromosomes are then not a typical 

 representative of animal chromosomes. 



Beam: The Edinburgh group have recently shown significant changes 

 in both chromosome number and structure following therapeutic X-ray 

 therapy to patients with ankylosing spondylitis (58). Maximum 

 changes occurred after three days of exposure and decreased rapidly 

 thereafter. In other patients, well-marked changes were observed within 

 24 hours following a single dose of X-rays of 250 rads directed to 

 the spine. 



Goldstein: This is a discussion I hoped would come up, because so 

 much of the evidence on "mutagenic activity" of the compounds I am 

 talking about is based on chromosome breaks, and I found it very 

 difficult to evaluate. Why should compounds which are mutagenic in 

 the strict sense of the word be chromosome breakers at all? This is 

 not clear to me. One is struck with the fact that a number of com- 

 pounds, not alkylating agents, which produce what seem to be point 

 mutations, are also chromosome breakers. If there is a similar mech- 

 anism here, then one has to take seriously the evidence based on 

 plants. If there were a basis for throwing it out, we might feel happier 

 about a number of drugs. 



I would now like to make some general remarks about the extent 

 of exposure of human populations to exogenous compounds. We must 

 consider not only the potential genetic hazard of compounds that 

 have recently come into pharmacological use. We must recognize that 



