Immunohistochemical detection of proliferating cell nuclear antigen (PCNA, a cell proliferation marker [18]) at 12 wk demonstrated that PCNA-positive cells in G1/S phase were localized extensively in small adenomas, focal alveolar hyperplastic lesions, injured perivascular and peribronchial regions, and nodular lymphoid aggregates in adventitia of blood vessels. Here, PCNA is linked to adenoma.