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. genesis 45:17–20, 2007. © 2007 Wiley‐Liss, Inc.