SELENOO (Selenoprotein O) is a selenocysteine-containing (Sec) selenoprotein belonging to the oxidoreductase family, which is mainly localized in the endoplasmic reticulum (ER) membranes with multiple transmembrane structural domains. SELENOO plays important roles in a variety of cellular processes, including redox homeostasis and lipid metabolism and DNA damage repair. It plays an important role in a variety of cellular processes, including redox homeostasis, lipid metabolism, and DNA damage repair. SELENOO is aberrantly expressed in a variety of diseases, including cancer and neurodegenerative diseases. For example, in melanoma and lung cancer, the upregulation of SELENOO expression is associated with tumor progression and metastasis, which has become a hot topic in current research.