['Polo-like kinase 1 (PLK1) is one of the serine threonine kinases that plays a role in cellular proliferation by activating the CDC25C-CDK1 (cdc2) loop. We recently demonstrated that cdc2 expression is associated with the biological aggressiveness of malignant lymphoma of the thyroid. In this study, we investigated PLK1 expression in thyroid lymphoma in order to elucidate its physiological significance in this disease.', 'We immunohistochemically investigated PLK1 expression in 46 cases of thyroid lymphoma and 10 cases of chronic thyroiditis.', 'Normal follicular cells did not express PLK1, whereas follicular cells in those in chronic thyroiditis and malignant lymphoma demonstrated a high level of PLK1 expression. In lymphocytes in chronic thyroiditis with strong lymphocyte infiltration, PLK1 was weakly expressed. In malignant lymphoma, PLK1 expression was observed in all cases, but PLK1 was more strongly expressed in cases with high cell proliferative activity and with cdc2 expression.'] Is polo-like kinase 1 ( PLK1 ) expression associated with cell proliferative activity and cdc2 expression in malignant lymphoma of the thyroid?