Hormones can force tumour growth, so drugs that interfere with that process, such as tamoxifen and aromatase inhibitors, are used as treatments.

However, up to a third of breast cancers are not hormone driven, so these drugs do not work and there are fewer treatments available for these patients.

http://www.medindia.net/news/Gene-Linked-to-Breast-Cancer-Identified-87804-1.htm