Mice hemizygous for this Osx1-GFP::Cre transgene are viable and fertile. The transgene carries both tTA under the regulation of the osterix (Sp7) promoter and, just downstream, a tetracycline responsive element (TRE; tetO)-controlled GFP/Cre fusion protein. In the absence of the tetracycline analog doxycycline, EGFP-Cre fusion protein expression is restricted to the osteoblast lineage throughout embryonic and early postnatal development. Fusion protein activity is largely absent from chondrocytes. When these transgenic animals are mated to transgenic strains that carry loxP-flanked (floxed) conditional alleles, Cre-mediated recombination of the floxed allele in the double mutant animals is placed under the regulation of doxycycline (dox) such that dox adminstration prevents fusion protein expression and recombination. The donating investigator suggests that the mice be maintained on dox-treated water to avoid incidental effects of tTA expression (e.g., malocclusion). The Jackson Laboratory maintains this strain on water containing doxycycline (final concentration of 1 mg/mL; Medisca NDC-38779-0434-4) and glucose (final concentration of 5 mg/mL) [November 2019]. The Jackson Laboratory 2024, used with permission.