A targeting vector containing a GFP/cre/ERT2 (green fluorescent protein, cre recombinase and estrogen receptor 1 tamoxifen binding domain fusion gene and an FRT flanked PGK-NEO selection cassette), was inserted into the translation initiation codon of exon 2 of the Prg4, proteoglycan 4 (megakaryocyte stimulating factor, articular superficial zone protein). The resulting chimeric animals were crossed to FLP recombinase expressing mice (ACT-FLPe) mice (on an unknown genetic background) to remove the Frt flanked PGK-neo selection cassette.