But that was one approach and then the Kleins in Stockholm, Georg and Eva, were trying another approach. It was actually suggested by Lederberg and they accepted it, and developed then the method. It was the following. You could induce... you could take a mouse, make an F1 hybrid.  In the F1 hybrid you induce a tumor and the tumor will have... it is a heterozygote it will have antigens of the Parent 1 and Parent 2 - both of them. And then you take the F1 tumor and transplant it on to one of the parents. The parent... now the F1 hybrid has antigens from both parents. The parent will recognise the antigens of the other parent and will mount an immune response against it, and it will destroy the tumor... it will of course not react against its own antigens. Well, it will destroy the tumor, but if there is a variant... genetic variant... if one cell of the tumor will change its antigens by whatever means, maybe somatic crossing over, maybe mutation or some... by some means... if it changes the antigens, that that cell will not be rejected. So you have a very good system for selecting very rare variants of the tumor. And they tried it and it seemed to work, and they could show that by selecting these variants you could lose some antigens.

Now, what were the antigens? The antigens were so-called H2 antigens. Now, I'll explain what is H2 a little bit later, but let's just say there were some antigens, and these antigens happened to be expressed on both red blood cells like normal blood groups, but also on other tissues and including the tumors... whichever tissue it was derived from. So it was a change of H2 antigens that they were observing, and now they were doing all kinds of what is lost and under what conditions and so on. And this was the system that Hašek gave me. In retrospect I think it somehow resembled... reminded him of what he tried to do originally. He called that at the beginning of his experiment, what he was trying to achieve was vegetative hybridisation. It's like somatic hybridisation. He was putting bodies together and trying to influence one by the other. So... and the fusion of cells was kind of vegetative hybridisation you could say. So it... I don't know. He never told me this or... but this is my sneaking impression that that's how he... why he favoured that system. But anyway somatic cell genetics at that time was already the catchword of the time because everybody expected hope for... hopes... great achievements from this area. And so he wanted somebody to work on that so I was the one who was supposed to do that.