Synonyms: 19-Nor-17a-pregna-4,9-dien-20-yn-3-one,21-chloro-17-hydroxy- (7CI,8CI); 17a-Chloroethynyl-17-hydroxy-13b-methylgona-4,9-dien-3-one; 17a-Chloroethynyl-17b-hydroxy-19-nor-4,9-androstadien-3-one; 17a-Chloroethynyl-17b-hydroxy-4,9(10)-estradien-3-one; 17a-Chloroethynyl-17b-hydroxy-4,9-estradien-3-one; 17a-Chloroethynyl-19-nor-4,9(10)-D-androstadiene-17b-ol-3-one; 17a-Chloroethynyl-19-nor-androsta-4,9(10)-diene-17b-ol-3-one; 17a-Chloroethynyl-4,9-estradien-17b-ol-3-one;21-Chloro-17-hydroxy-19-nor-17a-pregna-4,9-dien-20-yn-3-one; Ethynerone; MK 665