USP22 and lung cancer: Ubiquitin-specific protease 22 (USP22), a putative cancer stem cell marker, is frequently upregulated in cancers, and USP22 overexpression is associated with aggressive growth, metastasis, and therapy resistance in various human cancers including lung cancer. However, USP22 gene amplification seldom occurs, and the mechanism underlying USP22 upregulation in human cancers remains largely unknown.