Alveolar type II (AT2) cell dysfunction contributes to a number of significant human pathologies including respiratory distress syndrome, lung adenocarcinoma, and debilitating fibrotic diseases, but the critical transcription factors that maintain AT2 cell identity are unknown. Here we show that the E26 transformation-specific (ETS) family transcription factor Etv5 is essential to maintain AT2 cell identity. Deletion of Etv5 from AT2 cells produced gene and protein signatures characteristic of differentiated alveolar type I (AT1) cells. Consistent with a defect in the AT2 stem cell population, Etv5 deficiency markedly reduced recovery following bleomycin-induced lung injury. Lung tumorigenesis driven by mutant KrasG12D was also compromised by Etv5 deficiency. ERK activation downstream of Ras was found to stabilize Etv5 through inactivation of the cullin-RING ubiquitin ligase CRL4 COP1/DET1 that targets Etv5 for proteasomal degradation. These findings identify Etv5 as a critical output of Ras signaling in AT2 cells, contributing to both lung homeostasis and tumor initiation.A lveolar type II (AT2) cells are a stem cell population that self renews and differentiates into alveolar type I (AT1) cells during lung homeostasis or in response to injury (1, 2). They also give rise to lung adenocarcinoma induced by oncogenic Kras (1, 3-5). AT2 cells are derived from bipotent progenitor cells at the sacculation stage (1). In the adult lung, mature AT2 cells also secrete surfactant phospholipids to maintain normal alveolar function. Sustained Kras activity stimulates the self-renewal of AT2 cells, which eventually leads to abnormal tissue growth (1). An AT2 gene-expression signature has been reported (6, 7), but it is unclear how the identity of AT2 cells is specified and maintained.Given that Ras/MAPK signaling is essential for the self-renewal of AT2 cells and for the initiation of lung adenocarcinoma, identification of the transcription factors that are activated by Ras in AT2 cells is a step toward defining the transcriptional programs underlying AT2 stemness. The PEA3 subgroup of the ETS family of transcription factors, comprised of ETS transcription variants 1, 4, and 5 (Etv1, Etv4, and Etv5) (8), is known to be engaged by Ras/MAPK signaling. Etv4 and Etv5 are expressed in distal lung epithelium during development and are involved in branching morphogenesis and epithelial cell differentiation (9-11). Alveolar epithelial cells in the adult lung also express Etv5 (7, 12), but a critical role for Etv5 in this setting remains elusive.PEA3 transcription factors are also implicated in tumorigenesis. Overexpression of ETV1 or ETV4 has been linked to prostate cancer (13,14), and stabilization of ETV1 by mutant, active tyrosine kinase receptor KIT is thought to drive an oncogenic program in gastrointestinal stromal tumors (GIST) (15). ETV1, ETV4, and ETV5 are labile proteins because of their regulation by the ubiquitin-proteasome system. In GIST, ETV1 protein is stabilized by activation of the Ras/MAPK pathway through an unkno...