In the opening ‘question and answer’ chapter we blithely asserted that most folk know that genetic material is made of DNA – the stuff of heredity – and that it is damage to DNA (mutations) that cause cancer. As we noted, these gigantic molecules are made up of huge numbers of four small chemical units (the bases A, C, G and T), linked together in two chains. In humans there are about 3,000 million bases in each chain. It is the sequence of these letters in DNA that forms a code telling the cellular machinery which proteins to make – proteins being the things that do all the work and hence make individual cells and animals what they are.