What does ER-positive mean?

• A. Mutation in estrogen receptor gene
• B. Absence of estrogen receptor
• C. Overexpression of ER protein causing resistance
• D. Tumor growth driven by estrogen

Answer: (D)

ER-positive means breast cancer cells express estrogen receptors, and estrogen binding to these receptors promotes their growth. This receptor-driven growth explains why tumors classified as ER-positive often respond to endocrine therapies that block estrogen signaling, such as tamoxifen or aromatase inhibitors. The statement describing tumor growth driven by estrogen best captures the functional consequence of having the receptor: estrogen signaling fuels the cancer’s proliferation.

So, the other options don’t fit because ER positivity is not about a mutation in the receptor gene, nor about the receptor being absent, and it isn’t defined by overexpression causing resistance. It’s about the presence of the receptor and the tumor’s growth being influenced by estrogen.