Uncontrolled proliferation is a hallmark of cancer. In breast cancer, the most widely practiced method for comparing proliferation between tumor samples involves the immunohistochemical assessment of Ki67 antigen (also known as antigen identified by monoclonal antibody Ki-67 [MKI67]), a nuclear marker expressed in all phases of the cell cycle other than the G0 phase. Potential uses include prognosis, prediction of relative responsiveness or resistance to chemotherapy or endocrine therapy, estimation of residual risk in patients on standard therapy and as a dynamic biomarker of treatment efficacy in samples taken before, during, and after neoadjuvant therapy, particularly neoadjuvant endocrine therapy. Increasingly, Ki67 is measured in these scenarios for clinical research, including as a primary efficacy endpoint for clinical trials, and sometimes for clinical management.
Reference: Dowsett M, Nielsen TO, A’Hern R et al. Assessment of Ki67 in breast cancer: recommendations from the International Ki67 in Breast Cancer Working Group. J Natl Cancer Inst 2011;103:1656–1664.