A construct comprising the following elements was inserted into the host locus: CAG promoter, loxP site flanked STOP cassette, transcription start site, sequence for FLAG and Myc epitope tags, cDNA for the A isoform of the mouse progesterone receptor gene Pgr, and poly(A) features. This creates a conditional-ready allele where the inserted gene is only expressed after removal of the STOP cassette through cre-mediated recombination. (J:243133)