The Ki-67 protein (also known as MKI67) is a cellular marker for proliferation, and can be used in immunohistochemistry. It is strictly associated with cell proliferation. During interphase, the Ki-67 antigen can be exclusively detected within the cell nucleus, whereas in mitosis most of the protein is relocated to the surface of the chromosomes. Ki-67 protein is present during all active phases of the cell cycle (G1, S, G2, and mitosis), but is absent in resting (quiescent) cells (G0). Cellular content of Ki-67 protein markedly increases during cell progression through S phase of the cell cycle. In breast cancer Ki67 identifies a high proliferative subset of patients with ER-positive breast cancer who derive greater benefit from adjuvant chemotherapy.