A family of mutants overexpressing the Serratia marcescens extracellular nuclease has been known for decades. A number of these alleles are characterized here at the molecular level, and the mutant genes are identified, yielding a likely model for their phenotype. The known mutations exert their effect indirectly on nucA expression by elevating the basal SOS response of the cell. Mutations have been found in xerC and uvrD, both of which result in partial SOS induction. A classic nuc^su allele, that of strain W1050, is also likely to be in xerC.