d and not in others? Big Blue transgenic rodents with recoverable mutation target genes make use of the well-characterized bacterial lacI gene as a mutational target gene. The lacI gene, part of the lLIZ shuttle vector, is present in approximately 40 copies per chromosome in the mouse and rat. Mutations are efficiently assessed through the recovery of the l genome (Figure 1). High molecular weight genomic DNA is isolated from any selected Big Blue transgenic rodent tissue, which can be facilitated with the RecoverEase DNA isolation kit, and the l genomes are packaged into phage particles by Transpack lambda packaging extract.** The l particles are then plated on a specially constructed E. coli host in the presence of the chromogenic compound X-gal. Phages carrying the mutated lacI gene produce blue plaques, whereas the wild-type gene yields colorless plaques.

The ratio of blue to clear plaques indicates the mutant frequency, which in most tissues varies between 2 to 6 x 10^-5, although it is often lower in the rat than the mouse. Subsequent sequencing of the mutant lacI genes reveals the specific nature of the DNA change at the molecular level; when a sufficient number of mutants have been sequenced, a mutational spectrum results. Mutational spectra are extremely useful in assessing the mechanism of mutation. DNA sequencing also permits the mutant frequency to be adjusted by allowing a correction for clonal expansion in which an individual mutation is recovered multiple times because of cell division.

Tissue Specific Effects

Researchers who have designed studies to assess mutations in a range of tis
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