In 2007, an estimated 22,300 Canadian women were diagnosed with breast cancer, and 5,300 women died of the disease. To kill cancer cells, breast cancer patients undergo combinations of surgery, drug therapies, chemotherapy, and radiation. In spite of these aggressive treatments, certain cancer cells may not be completely eradicated and tumours may start growing again (relapse). In the event of breast cancer relapse, the prognosis is generally much worse than it was at the initial onset of the disease, and available drugs eventually become ineffective. It has been discovered that in breast cancer, only a small group of cells – called breast cancer tumour-initiating cells – can keep growing for a long period of time, while the other “”regular”” cancer cells cannot sustain themselves long term. With a better understanding of these aggressive tumour-initiating cells, researchers could design new drugs that target this special group of cells, and focus less on the cancer cells that will eventually stop growing on their own. Preliminary evidence suggests that tumour-initiating cells require a protein called YB-1 in order to grow and form tumours. Studies also show that patients whose breast cancers produce YB-1 have a higher chance of relapsing. Karen To is investigating whether she can stop the growth of tumour-initiating cells by blocking the production of YB-1. If this particular factor is proven essential for the growth of the tumour-initiating cells, drugs could then be designed to remove this protein. To’s research will contribute to the understanding of the small group of breast cancer cells responsible for maintaining tumour growth. Ultimately, this knowledge could lead to improved ways to treat this devastating disease.