Definition

A subtype of hormone receptor-positive breast cancer. Luminal B breast cancer cells typically have estrogen receptor (ER) proteins but not progesterone receptor (PR) proteins inside them. The cancer cells may or may not have the HER2 protein on their surface. Luminal B breast cancer cells often have high levels of a protein called Ki-67, which is found only in cells that are dividing. Luminal B breast cancers tend to grow more quickly and have a slightly poorer prognosis than luminal A breast cancers.