The ubiquitin-associated protein 2-like () gene remains poorly studied in human and mouse development. UBAP2L interacts with the Polycomb group protein B lymphoma Mo-MLV insertion region 1 homolog (BMI1) and determines the activity of mouse hematopoietic stem cells Here we show that loss of leads to disorganized respiratory epithelium of mutant neonates, which die of respiratory failure. We also show that overexpression leads to epithelial-mesenchymal transition-like phenotype in a non-small cell lung carcinoma (NSCLC) cell line. is amplified in 15% of human primary lung adenocarcinoma specimens. Such patients express higher levels of and show a reduction in survival when compared with those who do not have this gene amplification. Supporting a possible role for in lung tumor progression, NSCLC cells engineered to express low levels of this gene produce much smaller tumors than wild-type control cells. Together, these results suggest that contributes to epithelial lung cell identity in mice and that it plays an important role in human lung adenocarcinoma.-Aucagne, R., Girard, S., Mayotte, N., Lehnertz, B., Lopes-Paciencia, S., Gendron, P., Boucher, G., Chagraoui, J., Sauvageau, G. is amplified in a large subset of human lung adenocarcinoma and is critical for epithelial lung cell identity and tumor metastasis.