eLife assessment
By developing a novel method for detecting genetic variants associated with germline mutation spectrum variation, this important study identifies a new "mutator" locus in a population of inbred mouse strains, although the causal gene(s) and allele(s) within this locus remain uncertain. The authors further demonstrate that this new mutator locus interacts epistatically with a previously identified mutator allele on C>A mutation rate, showcasing the complexity of the genetic basis underlying variation in mutation rate and spectrum. Evidence for major findings in this paper is convincing, and the new method has the potential to be applicable to a variety of experimental systems and natural populations.