Selection of membrane progestin receptor-transfected cells for receptor characterization and signal transduction studies. Binding of [3 H]-P4 to plasma membranes of MDA-MB-231 cells stably transfected with human mPRα. A and B: Binding to ten subclones (a1-a10), obtained by limiting dilution, was assessed after three weeks selection. High [3 H]-P4 binding was associated with greater mPRα protein expression in the membrane fractions (insert) as assessed by Western blot analysis. C: [3 H]-P4 binding increased upon further selection for two weeks of the clones which had the highest binding in A&B (a1,a3,a5,a10). The receptor binding assay was conducted as described in Thomas et al., 2007. * P<0.05 compared to untransfected control cells (231-ve), Student's paired t test, N=4.