The Prg4, proteoglycan 4 (megakaryocyte stimulating factor, articular superficial zone protein), gene encodes the lubricin protein, a large proteoglycan that acts as a lubricant at the surface of cartilage of articulating joints. Prg4 also encodes for the superficial zone protein, megakaryocyte stimulating factor, and hemangiopoietin. These mice carry a targeted mutation (knock in) allele in which a GFP/cre/ERT2 fusion gene has been inserted into exon 2 of the Prg4 gene. Mice that are homozygous for the targeted mutation are viable and fertile, but exhibit a loss of the superficial-most layer of articular chondrocytes and displayed synoviocyte hyperplasia in joints. Expression of the GFP/cre/ERT2 fusion gene mimics expression of the endogenous Prg4 gene. When these Prg4GFPCreERt2 mice are bred with mice containing a loxP flanked sequence of interest, tamoxifen-inducible, Cre-mediated recombination will result in deletion of the flanked sequences in Prg4 expressing cells. The Cre-ERT2 fusion protein consists of Cre recombinase fused to a triple mutant form of the human estrogen receptor which does not bind its natural ligand (17?-estradiol) at physiological concentrations but will bind the synthetic estrogen receptor ligands 4-hydroxytamoxifen (OHT or tamoxifen) and, with lesser sensitivity, ICI 182780. Restricted to the cytoplasm, Cre-ERT2 can only gain access to the nuclear compartment after exposure to tamoxifen. To counteract the mixed estrogen agonist effects of tamoxifen injections, which can result in late fetal abortions in pregnant mice, progesterone may be coadministered.