personally I think it’s the discovery that cancer is a genetic disease, that means that there are mutations happening in the DNA. This means that if we understand all the different mutations leading to cancer, than we should be able to cure it
This is a great question and I’ve been thinking about it a lot. I think my answer is going to be the discovery of the molecular structure of DNA in the 1950s. It wasn’t specifically cancer research – it was basic biological research (like the work I do), but without this work we wouldn’t be able to understand how mutations and cancer arise.