The effect of letrozole, a non-steroidal aromatase inhibitor, also leads to apoptosis of hormone-dependent breast cancer cells and, as in the case of exemestane, this effect is stronger in MCF-7 cells than in MCF-7/DOX, which may be explained by lower concentrations of the pro-apoptotic protein BAX in the MCF-7/DOX cell line (a higher Bcl-2/BAX ratio). The gene discussed is PROS1; the disease is breast cancer.