One of the most common drugs used to treat breast cancer is tamoxifen. It works by blocking the activity of estrogen, reducing the growth of cancer cells. Despite the success of tamoxifen, many cancers eventually become resistant to its effects. The resistance has been shown to be linked to mitochondria, small cellular structures that are the source of cellular energy. Specifically the resistance was linked to the activity of a single gene - NQ01. Importantly, inhibitors of NQ01 exist, and testing showed that tamoxifen resistance was reversible in a lab model, opening up the possibility of reversing drug resistance in humans.