DNA Topoisomerases are a unique class of enzymes that are responsible for the control and modification of DNA topology (tertiary structure); one example of their function is the transformation of chromosomes from a supercoiled condensed state to uncoiled strands and vice versa. They are of interest both for their intrinsic role in cell biology (i.e. control of cell division) and also because some anticancer drugs, such as camptothecin, act by inhibiting their action.