In order to examine the relationship between cell proliferation and ERK1/2 activation in mouse lung tumors, double‐immunohistochemical staining for pERK1/2 and PCNA was performed in a total of 61 lesions (13 hyperplasias, 34 adenomas, and 14 adenocarcinomas) (see Fig. 5). The gene discussed is MAPK3; the disease is adenocarcinoma.