The notion of the probability of back mutation is introduced and the method of probability generating functions is used in order to simplify and unify the Holmquist's (1972) investigation of the effect of multiple hits on nucleotide differences between homologous DNAs. We obtain explicit expressions for the distribution of the number of hit nucleotide sites, the number of altered sites, and the number of differences between two homologous DNAs, as functions of the total number of hits. For the case where the hit rate is a known function of time, we derive a formula for extending these results as functions of time.