Here, we report the first application and characterization of the cII locus as a mutational target for use with the Muta ™ Mouse system for quantifying somatic mutations in vivo. This locus can be analyzed for mutations using positive selection and is identical in sequence to the cII in the Big Blue ® Mouse. The cII displays similar spontaneous (5.5 ϫ 10 -5 ) and induced mutation frequencies when compared to the lacZ gene in the small intestine of MutaMice treated with ENU (N-ethyl-N-nitrosourea). After acute treatment with 250 mg/kg ENU (ip) the mutant frequencies were 127 ϫ 10 -5 at the cII and 147 ϫ 10 -5 at the lacZ loci, reaching a maximal mutant frequency 10 days posttreatment and remaining constant thereafter. These data prove that this transgene is genetically neutral, conferring neither selective advantage nor disadvantage on the host cells. The cII dose response curve was linear (R 2 ϭ 0.93) comparable to the lacZ after treatments with 0, 50, 150, or 250 mg/kg ENU. Use of the cII locus (0.3 kb) addresses the single most significant drawback associated with the Muta-Mouse system, namely the inability to obtain sequence spectra efficiently, due to the large size of the lacZ gene (3.0 kb). Moreover, a less obvious application, but nevertheless of considerable importance, is the easy identification of jackpot mutations, without sequencing. The cII, identical in both sequence and origin on the transgenic constructs used in producing the Big Blue and Muta-Mouse systems, provides the first transgenic locus common to the two widely used in vivo mutagenesis assays.