Based on current incidence rates, 12.4% of women born in the United States today will develop breast cancer at some time during their life.¹ At the time of diagnosis, breast cancer is generally considered local, but eventually approximately 20% of patients will experience either locoregional or distant disease recurrence.²

Approximately 75% of breast cancers rely on ER signaling to grow and survive.² Endocrine therapy, which blocks the growth-promoting activity of estrogen, represents the primary intervention for early- and advanced-stage ER-positive breast cancer.³ However, some patients do not respond to endocrine therapy (ie, de novo resistance), and some patients who initially respond to therapy have disease that progresses during therapy (ie, acquired resistance).³