Breast cancers are defined by their drivers -- estrogen and progesterone receptors (ER and PR) and HER2 are the most common, and there are drugs targeting each. When breast cancer has an unknown driver, it also has fewer treatment options -- this aggressive form of breast cancer without ER, PR or HER2, which was thought not to be driven by hormones, is known as triple negative. A decade ago, work at the University of Colorado Cancer Center added another potential driver to the list -- the androgen receptor -- and this week marks a major milestone in a clinical trial targeting this cause of breast cancer growth