Mouse Annexin V (also known as ANXA5, Annexin A5, placental anticoagulant protein I (PAP-I), lipocortin V, or anchorin CII) is a calcium-dependent phospholipid-binding protein belonging to the annexin family, which plays important roles in membrane organization, cellular signaling, inflammation, and regulation of apoptosis. In mice (Mus musculus), Annexin V binds with high affinity to phosphatidylserine exposed on the outer leaflet of the plasma membrane during early stages of apoptosis or cellular damage in the presence of calcium ions. Because of this property, FITC-labeled mouse Annexin V (Annexin V–FITC) is widely used in flow cytometry and fluorescence microscopy assays to detect and quantify apoptotic cells. Conjugation with fluorescein isothiocyanate (FITC) enables fluorescent labeling of cells exposing phosphatidylserine, allowing sensitive identification of early apoptotic cells, typically in combination with viability dyes such as propidium iodide (PI) or 7-AAD to distinguish early apoptosis from late apoptosis or necrosis. In mouse research, Annexin V–FITC assays are extensively used in studies of immune cell apoptosis, cancer biology, drug-induced cytotoxicity, neurodegeneration, and inflammatory diseases. Because mice are the most widely used genetic model organism in biomedical research, Annexin V–FITC staining is a standard tool for investigating cell death pathways, evaluating therapeutic responses, and studying host–pathogen interactions. As a biomarker of apoptosis and membrane remodeling, mouse Annexin V plays a critical role in experimental studies of cell survival, immune regulation, and disease pathogenesis.