High levels of the Ki-67 protein are associated with poor prognosis in early breast cancer patients, but it may not able to predict which patients will benefit from additional chemotherapy. The percentage of proliferating cells in a tumor is a predictor of breast cancer prognosis. The level of Ki-67 expression is sometimes used as a measure of cell proliferation. Some studies have suggested that breast tumors with a high percentage of cells expressing Ki-67 are more responsive to chemotherapy.