There'd been a lot of work done on conditional lethals of bacteriophage, in that people had started to identify temperature-sensitive mutants. And later on the amber mutants were identified as conditional lethals… finding all the genes of an organism. While we were working in this I had started to isolate a number of mutants in bacteriophage T4 called minutes. I worked extensively on these minutes during 1960 when I could show that there were lots and lots of different ones. And they were difficult to work with, but we did in fact map quite a large number of ones. I didn't have the sense to see that these were just leaky mutants – that is, almost conditional, almost lethals. And that had I incubated some of them at a lower temperature I would have discovered that quite a lot of them were temperature-sensitive, and I just hadn't gone up high enough to knock them out completely. Later we did check our minute collection for temperature sensitivity and found indeed that they were temperature-sensitives. But that whole concept of conditional lethals opened up genetics in a most remarkable way. And in fact became the basis for a considerable amount of genetics since that time, and gave rise I think to the concept, which became important later, that Seymour used to call genetic dissection. The… the DNA mutants were of course a paradigm for me. No doubt other people will have their own examples. But that showed exactly what you had to accomplish in any large-scale genetic experiment, which was roughly speaking; isolate as many mutants as you can, classify them by complementation, so that you've got all the ones in the same gene, and then study them as deeply as you can in order to find out what they do. And that has… has flowed through all my work since the early '60s – just that general approach.