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Ki-67 Expression

Meaning

Ki-67 expression is a clinically relevant biomarker that quantifies the fraction of cells actively engaged in the cell cycle, excluding those in the quiescent G0 phase. The Ki-67 protein is present during the G1, S, G2, and M phases of the cell cycle but is absent in resting cells. High Ki-67 expression, typically determined by immunohistochemistry, is directly correlated with a high cellular proliferation rate and is used as a prognostic factor in hormone-sensitive cancers, such as breast cancer. A high proliferation index often suggests a more aggressive tumor phenotype and is a critical consideration in treatment planning.