We inactivated TP53 and/or RB1 by siRNA mediated knockdown in breast cancer cell lines varying with respect to ER/PgR and Her-2 status as well as TP53 and RB1 mutation status (MCF-7, T47D, HTB-122 and CRL2324) and determined effects on cell cycle arrest, apoptosis and senescence with or without concomitant treatment with doxorubicin. The gene discussed is PGR; the disease is breast cancer.