At the time of diagnosis, the majority of breast cancers are categorized as estrogen-receptor positive, or hormone sensitive, which means their cancerous cells may need estrogen to grow. Patients with this type of cancer often respond favorably to treatments called aromatase inhibitors, like tamoxifen, which cause cell death by preventing estrogen from reaching the cancerous cells. Over time, however, the disease often becomes resistant to estrogen deprivation from the drugs—making treatment options more limited.
