The initiating oncogenic event in half of human lung adenocarcinomas is still unknown, a fact that complicates the development of selective targeted therapies. Researchers have demonstrated that the expression of an endogenous Braf kinase-inactive isoform in mice triggers lung adenocarcinoma in vivo, indicating that BRAF-inactivating mutations are initiating events in lung oncogenesis. The paper indicates that the signal intensity of the MAPK pathway is a critical determinant in tumor development.