A splice acceptor site was inserted immediately before the Dre and the mutated human progesterone receptor (PGR) consisting of the Ru486 responsive mutant hormone-binding domain hPR891 was fused to the N-terminus of Dre sequence followed by a bovine growth hormone (BGH) polyadenylation signal and PGK-Puro selection cassette. The targeting vector was inserted into a Gt(ROSA)26Sor site by homologous recombination in ES cells.