DRUG ACTION UPON GENES I43 



an effect, amongst the n^ore entertaining of which is the 

 possibiHty that the inhibitors may in fact be substances 

 causing a more rapid turn-over of the growth-promoting 

 substance in the cells. It is a characteristic of this bal- 

 ance of control that it is adjusted so that normally mu- 

 tations permitting escape from this control are infrequent. 

 But when additional foreign growth inhibitors are present, 

 mutation appears to be more frequent, and may involve 

 not only escape from the foreign growth inhibitor but 

 also from the normal endogenous growth inhibitors. 

 Cells which suffer such mutations may give rise to 

 tumours. There is a good deal of evidence available now 

 suggesting that mutations, whether they be of nuclear 

 genes, or of plasma genes, may commonly take place 

 under the action of drugs, such as penicillin, the sulphon- 

 amides, arsenicals, etc. and thus give rise to strains of 

 cells (usually of micro-organisms) which are resistant 

 to the drug concerned. 



If we are seriously to adopt the point of view which 

 has just been suggested, namely, that mutation may be 

 a fairly common event under appropriate conditions, we 

 must reconsider our attitude towards the stability of 

 genes. It has usually been supposed that genes are re- 

 markably stable bodies. This point of view has arisen be- 

 cause the occurrence of mutations is normally a very 

 infrequent process. However, when we consider the prob- 

 able chemical composition of genes, i.e. a combination 

 of deoxypentose nucleic acid and protein, there appears 



