EARLY studies of the effects of acridines on amphibian egg development tended to show that RNA synthesis is required for neural induction1. Little could be concluded from these experiments, however, because the acridines available at that time were a complex mixture of substances of unknown mode of action. Acridines have since been found to bind to and distort the DNA molecule by intercalation of dye molecules between base pairs2. One of the acridines, ethidium bromide, is now used to measure the number of superhelical turns in circular mitochondrial DNA molecules3–5.