On the origin of unusual mutations recovered from the lacI transgenic assay

Amongst approximately 25,000 mutants recovered from tissues of the lacI mouse and rat transgenic mutation assay, we identified seven mutants that carry changes that are unlike the majority of mutations that are normally recovered in these systems. The recovered mutants feature replacements and insertions of sequences that originate in the animal's genome, in the bacteriophage lambda construct that harbors the lacI gene, and in the genome of the E. coli plating host. These mutants demonstrate that mutations resulting from diverse mechanisms, in addition to the normal point mutations, can be recovered. In addition, the data indicate that such mutations may often not be of animal origin.