An AI is a type of drug that prevents an enzyme called aromatase from making estrogen. Blocking aromatase lowers the amount of estrogen in the body, which may stop the growth of cancer cells that need estrogen to grow. AIs are a type of hormone therapy used to treat HR+, HER2- breast cancer.

CDK4 and CDK6 are proteins that can cause cancer cells to spread.

Estrogen is a hormone that helps control sexual and reproductive health.

A hormone receptor is a protein found either on or inside the cell that binds to a specific hormone, such as estrogen.

HR+ breast cancer is cancer with cells that have hormone receptors. HR+ cancer cells need estrogen or progesterone to grow.

Hormone therapy is treatment that is used for certain types of breast cancer to add, block, or remove hormones. Hormone therapy is also referred to as endocrine therapy.

HER2 is a protein found in both healthy cells and cancer cells. HER2 plays a role in normal cell growth. In cancer, HER2 amounts are altered, which can cause the growth of cancer cells.

HER2 status means the amount of HER2 proteins found on breast cancer cells. 
If a lot of HER2 proteins are found, the cancer is called HER2 positive (HER2+). If no or very few HER2 proteins are found, the cancer is called HER2 negative (HER2-).

HER2- breast cancer is a type of cancer where cancer cells have no or very low levels of the HER2 protein. HER2 can cause cancer cells to grow quickly.