Interferon-induced transmembrane protein 3 (IFITM3) is a small transmembrane protein that plays a crucial role in various cellular functions. It is ubiquitously expressed in most cell types, primarily localized in endosomal and lysosomal membranes, and can also be found in plasma and endoplasmic reticulum membranes. IFITM3 is best known for its antiviral activity, as it can inhibit the entry of a broad spectrum of viruses, including influenza A virus, Zika virus, coronaviruses, and others, by preventing viral membrane fusion with host cells. In addition to its antiviral function, IFITM3 is involved in cell adhesion, stem cell migration, and immune regulation. It has also been implicated in tumor development and progression, as well as in the pathogenesis of diseases like Alzheimer's Disease. The expression of IFITM3 is regulated by interferons, and genetic variations in the IFITM3 gene have been associated with susceptibility to viral infections.