Definition

A subtype of hormone receptor-positive breast cancer. Luminal A breast cancer cells typically have estrogen receptor (ER) and progesterone receptor (PR) proteins inside them and a small amount or none of a protein called HER2 on their surface. Luminal A breast cancer cells also tend to have low levels of a protein called Ki-67, which is found only in cells that are dividing. Under a microscope, luminal A breast cancer cells look more like normal cells than other subtypes of breast cancer. They tend to grow slowly and have a good prognosis. Most breast cancers are luminal A.