The transgene consists of a tamoxifen-inducible cre recombinase driven by the 2.3-kb mouse Col1a1, collagen, type I, alpha 1, promoter. The transgene insert contains a fusion product involving cre recombinase and a mutant form of the mouse estrogen receptor ligand binding domain. The mutant mouse estrogen receptor does not bind natural ligand at physiological concentrations but will bind the synthetic ligand, 4-hydroxytamoxifen. Restricted to the cytoplasm, the cre/Esr1 protein can only gain access to the nuclear compartment after exposure to tamoxifen. Inducible cre recombinase activity is detected in the osteoblasts of the long bones of the limbs and in the ribs, vertebrae and calvaria in 18.5 embryonic day aged and 18 day old mice. Inducible cre recombinase activity was not detected in chondrocytes.