Talking in the Eagle one time, I knew that proflavine… because of my long interest in these damn acridine dyes and their attachment to DNA and all the measurements that had been made in the early '50s by people, though I began to wonder whether in fact there was not another hypothesis. And in fact by some very tortuous reasoning which of course clarified the moment one said it, I said that, 'What would it be like if there were not only base substitutions but base additions and deletions?' And I suddenly realised that you could get that if you stuck proflavine into the DNA, right, or if it went between the base pairs as people did it – intercalation – and you could have a thing where the DNA thought it… this was another base and stuck an extra one in, okay, or it thought on the other side, if it was copying the other strand and made a deletion – that there was a base running free. So the whole idea then came of some connection between additions and deletions, all right? And this then said of course that acridine mutants would have a drastic effect.