The mutation affecting DNA polymerase activity in the mutant strain isolated by De Lucia and Cairns is located between metE and rha, at approximately 75 minutes on the Escherichia coli chromosome. The mutation is recessive to the wild-type gene in partial diploids. It is an amber nonsense mutation which responds to the suppressors Sul+, Sull+ and Sulll+. Strains carrying the mutation are not deficient in carrying out genetic recombination. © 1969 Nature Publishing Group.