Ki-67 is a marker for what cellular process?

• A. Nuclear proliferation
• B. Cytoplasmic enzyme activity
• C. Membrane receptor expression
• D. Secretory protein

Answer: (A)

Ki-67 is a marker of cellular proliferation. It is a nuclear protein that appears in cells actively cycling through the cell cycle (G1, S, G2, and M phases) and is absent in resting cells (G0). Because it labels nuclei only in proliferating cells, the Ki-67 staining index reflects the growth fraction of a cell population. This makes it useful in pathology to assess tumor aggressiveness and monitor responses to therapies that affect proliferation. It is not related to cytoplasmic enzyme activity, membrane receptor expression, or secretory proteins, since Ki-67 resides in the nucleus and specifically marks actively dividing cells.