148 MUTATIONS 



tween the A and B cistrons must be such a hot spot, because this type 

 of mutation which cuts out the whole B cistron has been observed no 

 fewer than ten times. This is also true of many others, including small 

 deletions. 



Therefore, there are hot spots for the beginning and end of a dele- 

 tion. You can find nests of deletions within deletions. 



Novick: Do the spontaneous hot spots tend to be in the center of 

 the deletion hot spot? 



Benzer: No. There is no apparent correlation between hot spots for 

 point mutations and hot spots for deletion end points. 



Lederberg: Do you want to say just that — or that there is a 

 tendency for the breaks to occur, and then you get all combinations of 

 segments between those points of breakage? 



Benzer: Yes, many combinations. 



Lederberg: To generate the type of segment you have, would it be 

 sufficient to give the frequencies at which you find the beginning and 

 end and put them together in an appropriate combination, or, if it 

 starts here, does it have a particular reference ending there rather 

 than here? 



Benzer: I think I would want to say both. There are cases of the 

 beginning correlating with the end, and those are the ones I'm talking 

 about here — recurrent, identical deletions. But there are also many 

 cases where the beginning points look the same but the end points 

 are different. You see I can give you almost anything you want. I've 

 got a trunkful of mutants. 



Freese: How hot is such a beginning point or an end point when 

 compared to the hotness of point mutations? 



Benzer: We can estimate among the same set of mutants studied 

 for point mutations, where the hottest spontaneous spot has 500 re- 

 currences, the hottest deletion has of the order of 10. Approximately 1 

 in 20 spontaneous mutants is a deletion. 



Auerbach: But what if you take an end point only, not the whole 

 thing? It must be hotter. 



Benzer: It's not much different, since in the hottest cases the end 

 points are mostly the same. 



Lederberg: You define the terminus of this particular class of dele- 

 tions at this point. What is the temperature for point mutation at this 

 particular point? 



Benzer: There is no obvious correlation. Of course, it must be under- 

 stood that the terminus of a deletion is only defined in terms of two 

 sites. All you can say is that a deletion ends between one and the 



