Rearrangement of a conditional allele regardless of inheritance of a Cre recombinase transgene

To generate conditional gene knockouts in osteoblasts, we previously developed transgenic mice in which Cre recombinase cDNA was cloned downstream of a 3.6 or 2.3 kb fragment of the rat Col1a1 promoter (Col3.6-Cre and Col2.3-Cre, respectively). Col-Cre mice were bred with mice in which exon 4 of the Igf1 gene is flanked by loxP sites. Mating units were arranged such that either the male or the female breeder transmitted the Col-Cre transgenes. Progeny were evaluated for Cre-mediated Igf1 gene rearrangement. We found that the loxP-flanked Igf1 locus was rearranged in the absence of inheritance of the Cre transgene. The incidence was 50 and 28% with Col2.3-Cre and Col3.6-Cre females, respectively, and 15 and 18% with Col2.3-Cre and Col3.6-Cre males, respectively.