"We have to go back in time now to the 1940s. And remember that this is a time before the Watson-Crick discovery of structured DNA. And to know that DNA was understood to be the sort of essence of life and necessary to be duplicated in order for cells to divide, and for cells to live. One knew what the building blocks were, in there, because you could do it by just taking the thing apart and looking at the individual components. So the idea was, let's change the building blocks, slightly, just enough perhaps to fool a cancer cell or a bacteria or a virus into taking it up and then find that it's stuck and it can't get rid of it and it can't multiply."