Breast cancer is the most common cancer in women. There is an urgent need for new drugs with novel modes of effect on tumors. It is very important to develop compounds with high activity against tumor cells and low toxicity on normal epithelium. The development of new anti-tumor drugs based on steroidal compounds is a promising approach for obtaining agents that combine antiestrogenic, cytostatic and proapoptotic effects. The aim of the work is to study the mechanism of anticancer action of new steroidal compound, 3,20 (R)-dihydroxy-19-norpregnatriene (Kuz7).