Representative histology of NNK-induced lung tumors in mice. (A) Alveolar adenoma at low magnification. (B) High magnification of alveolar adenoma shown in A. (C) Carcinoma (middle and right side) in adenoma (left side of figure). (D) Invasive adenocarcinoma into pleura and mediastinal fat (invasive front shown by arrows). (E, F) Adenocarcinoma in adenoma for Ki67 staining. Only adenocarcinoma is highly positive for Ki67. Original magnification: A, 100X, B, C, 400X, D, F, 200X, E, 40X.