A and C, Competition curves of ligand binding to recombinant human mPRα on plasma membranes of transfected MDA-231 cells expressed as a percentage of maximum specific [3H]P4 binding. P4, progesterone; 18-CNP4, 3,20-dioxopregn-4-ene-18-carbonitrile; 19-OHP4, 19-hydroxyprogesterone; 1,4-pregnad, pregna-1,4-diene-3,20-dione; 4,6-pregnad, 4,6-pregna-4,6-diene-3,20-dione. For key to other steroid abbreviations see Table 1. Assays were repeated 3 times and similar results were obtained in each assay. B, Molecular structures of four synthetic progestins with high affinities for mPRα.