Ki-67 is a fundamental marker used in the field of oncology and pathology for assessing cellular proliferation. Its expression provides critical insights into the growth dynamics of tumor cells, making it an invaluable tool in both research and clinical settings.

Ki-67 is a nuclear protein that plays a pivotal role in cell proliferation. It is associated with ribosomal RNA transcription. 

The presence of Ki-67 is strictly limited to the active phases of the cell cycle—G1, S, G2, and M phases. It is not detected in quiescent cells (G0), which makes it an ideal marker for determining the proliferation status of cells.

In conclusion, Ki-67 is a critical biomarker for assessing cell proliferation in tumors, providing essential insights that influence the management and prognosis of cancer patients. Its role extends beyond diagnostic and prognostic applications into therapeutic decision-making and research on cellular kinetics.