190 MUTATIONS 



precise method there is for general purposes, wherever it could apply. 

 We will want to try to look for point mutations in mammalian cells 

 and cell cultures. Unfortunately, there have been very grave technical 

 difficulties connected with this. I think there are many people working 

 in this field and interested in mutation who seem to forget that we are 

 dealing with a diploid tissue and, in some cases, a hyperploid tissue. 

 Until we get around this particular barrier, we're not going to get very 

 far with point mutations. We can look for dominant mutants, and 

 there are some tricks we can do. 



Atwood: The scarcity of markers is a point. 



Auerbach: What about resistance mutations? 



Lederburg: Drug resistance in bacteria, for the most part, has proved 

 to be a recessive mutation. Frequently, drug resistance has been found 

 in mammalian tissues. It is often associated with changes in karyotype, 

 and I can think of no simpler explanation of this than the fact that it 

 would require a chromosomal reorganization to unmask a recessive 

 mutation in a heterozygous condition in mammalian cell culture, so we 

 have here an extremely complex situation which is not going to be 

 easily solved. 



Either we take pains to do genetically well-defined material in cell 

 culture, and that means mouse and not man, or we should take pains, 

 I think, to try at least one small trick, that is, to use male rather than 

 female cells, and hope to get sex-linked recessives which will at least 

 expose part of the genome for observation. Alternatively, we might 

 spend some effort in trying to find a haploid tissue that could be culti- 

 vated. This may or may not be a reasonable proposal. There have been 

 reports that the syncytial trophoblast in haploids might be used. There 

 may be other tricks we can use for getting this out. We may be able to 

 use monosomic tissues which are now beginning to become available in 

 man, and we can look for these in other experimental organisms as a 

 way of exposing at least single chromosomes in a haploid state, to the 

 point where we can look for point mutations in them. But as long as we 

 use diploid tissue, the prospects are very remote indeed. 



Glass: I want to differ with that point of view in one respect at least, 

 although in general I agree fully. I believe there is an additional possi- 

 bility that makes the outlook not quite so difficult, and that is the 

 possibility of using human tissue cultures from individuals who are 

 known to be heterozygous for recessive mutations. 



Atwood: Provided it is a marker that can be used at the cell level. 



Glass: That is the problem — to find the marker that can be detected 

 in individual cells. 



