PROLIFERATIVE DISEASES 



Stable strains can disturb some control and multiply preferen- 

 tially in the uncontrolled cells; this seems the most likely 

 interpretation of the Bittner mammary cancer factor, (ii) The 

 agents are subcellular units genetically of host origin. 



An attempt may be made to examine mouse leukaemia 

 from this second point of view, having particularly in mind 

 Medawar's speculations about the function of lymphocytes 

 as carriers of nucleic acid. We have already suggested that 

 there is immunological evidence for the possibility that in 

 the lymph nodes transfer of genetic qualities may occur 

 between lymphoblasts multiplying in proximity to each other. 

 The evidence from isotope experiments that DNA can be 

 transferred amongst lymphocytes provides a slight but 

 significant basis for this speculation. In turn this opens up 

 the possibility of providing a more definite picture of the 

 second type of operational virus. If it is accepted that, under 

 some circumstances, a lymphocyte (mesenchymal cell) can 

 take up functional DNA, so modifying its genotype that it is 

 now subject to growth-stimulating influence previously in- 

 effective, the possibility of an analogy in leukaemic processes 

 is obvious. 



We might suggest that certain cells on replication release, 

 in the fashion postulated by Medawar (1957), DNA-contain- 

 ing units which can be taken up by other growing cells of the 

 mesenchymal series. In what is probably a very small pro- 

 portion of such interactions, a fragment of DNA is built into 

 the genome (transduction) and as a result some control is 

 abrogated, just as may occur in a spontaneous mutation. 

 It is only necessary to add the further assumption that these 

 modified cells will, like a transformed pneumococcus, be able 

 in their turn to liberate the agent which converted the paren- 

 tal cell to the new type. It is stressed that the only virtue of 

 this formulation is that it brings the autonomous subcellular 

 unit, plasmagene, etc., of many previous writers into relation- 

 ship with phenomena observed in two other fields — the un- 

 expectedly long persistence of labelled DNA in populations 



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