 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 

Ki-67 is an excellent marker to determine the growth fraction of a given cell population.