estrogen-receptor-negative HER2-negative This means that the growth of the cancer isn’t driven by the hormones estrogen or progesterone, or by the HER2 protein. So, triple-negative breast cancer doesn’t respond to hormonal therapy medicines or medicines that target HER2 protein receptors. About 15% of breast cancers are triple-negative. Triple-negative breast cancers tend to be: considered more aggressive than hormone-receptor-positive or HER2-positive breast cancer, mainly because there are few targeted therapy medicines that treat triple-negative breast cancer; studies suggest that triple-negative breast cancer is more likely to spread beyond the breast and more likely to come back after treatment (recur) than other types of breast cancer