In late 1971 we were involved in a study of the interaction of radiation with matter and were trying to use measurements of radiation fluorescence in biological molecules to indicate how radiation affected living cells. It soon became apparent that we were working in the dark; the doses we used to get a significant signal were too large to be of interest for radiation biology and although the DNA molecule appeared to be the most likely target molecule we did not know which sort of events and which sort oflesions were the most important. We decided to alter our approach to see if we could find...