Breast cancer is the most common cancer in women, and the second most common cancer to cause death in women. Although there has been significant development in the treatment of breast cancer over the last two decades, current therapies are still very limited in their ability to control metastasis and relapse, especially for erbB2high breast cancer. The erbB2 gene is highly expressed in approximately 30% of breast cancers, and erbB2high breast tumours are known to be more aggressive and more resistant to therapy, usually showing early relapse and poor prognosis. Treatment of erbB2high breast cancer has long been a challenge in cancer research. Therefore in this project, we focus on the tumour-initiating cells (TICs) of erbB2high breast cancer. Accumulating evidence suggests that breast cancer involves TICs. Over the last decade, the TIC (or cancer stem cell) model of tumour propagation has become increasingly accepted. TICs are tumorigenic and have the properties of self-renewal and multi-potential differentiation. They are usually resistant to chemotherapy and radiotherapy, and therefore have the ability to re-initiate tumours, as well as to determine metastasis and relapse of the disease. Accordingly, specific therapies targeted at TICs may be potentially used for complete elimination of tumours, and can therefore significantly improve the survival and quality of life of cancer patients, especially for sufferers of the metastatic disease. The existence of TICs has been proved in many types of malignancies, including breast cancer.