50% positivity of Ki67 antigen refers to 50% of Ki67 expression positive cells in immunohistochemical staining, indicating high cell proliferation activity. Ki67 antigen is only expressed in the late G1, S, G2, and M phases of the cell cycle, not in the G0 phase where the cell nucleus is quiescent, and has a short half-life, which is an indicator of tumor proliferation activity. Ki67 positivity rate refers to the proportion of cells with positive Ki67 expression in immunohistochemical staining to the total tested cells. In different tumors, different positivity rates are used as the cut-off values for high and low expression, such as 20-30% for breast cancer, 25% for cervical cancer and gastric cancer, and the cut-off values need to be referred to the standards of different laboratories. A Ki67 positivity rate of 50% is generally considered as high expression, indicating that the tumor is actively proliferating, poorly differentiated, highly malignant, with high risk of recurrence and metastasis, and poor prognosis. Patients are advised to combine immunohistochemistry and other indicators to make a comprehensive judgment, and consult with specialists for treatment options in a timely manner.