The goal of this complementation experiment was to find out if the two different mutations are on the same cistron or on two different cistrons. We know that the mutation T4 rII UV375 is located on cistron B, our objective is therefore to find out whether mutation T4 rII AP211 is located on cistron A or B. We expected no plaques if both mutations were located in cistron B as complementa-tion would not happen. In contrast, we expected plaques on the plates if one mutation was located on