MITOTIC POISONS I35 



available. On theoretical grounds one would be inclined 

 to predict that most of the mitotic abnormalities which 

 are seen as a result of the action of a dose of nitrogen 

 mustard could originate as the result of the failure of one 

 or a few genes either to reproduce, or to function. The 

 reproduction, or functioning, of a single gene can prob- 

 ably be inhibited by combination with one molecule of 

 nitrogen mustard. Particularly significant in this con- 

 nection is some of the recent work of Herriot on viruses, 

 the reproduction of which he shows to be much more 

 sensitive to mustard than is any other biological process 

 so far examined. If it should prove to be true that the 

 most important site of action is upon the genes and that 

 the action is exerted by a small number of molecules of 

 nitrogen mustard, then quite exceptional methods will 

 be necessary to detect the exact site of action. Further- 

 more, the identification of the chemical action exerted 

 upon the gene will be very difficult to ascertain, partly 

 because of the quantities involved, and partly because 

 the reactions between the gene and the drug need not 

 necessarily be those which are regarded as in the main 

 course of chemical reactivity of the drugs concerned. 

 Where a few molecules only out of a relatively large 

 dosage can have a definitive action upon a biological proc- 

 ess, it is perfectly possible for the key reaction involved 

 to be amongst those which are normally classified by the 

 organic chemist as "side reactions", and be usually even 

 less understood than the characteristic reactions. 





