**9. Endocrine resistance for breast cancer**

ER is expressed in around 70% of breast malignancies and plays an important role in their genesis and progression. Because of the involvement of ER in ER+ breast cancer, endocrine treatments such as aromatase inhibitors (AIs), selective oestrogen receptor modulators (SERMs), and selective oestrogen receptor degraders are commonly used to treat these tumours (SERDs). While hormone treatments have been successful in avoiding recurrence, about 20% of these tumours acquire resistance to hormone therapies and will return.
