Cytokeratin 15 (CK15, encoded by KRT15) is a type I intermediate filament protein belonging to the keratin family. It is predominantly expressed in the basal and suprabasal layers of stratified epithelia, with particularly strong expression in the bulge region of the hair follicle, a well-characterized niche for epithelial stem cells. CK15 serves as a reliable marker for quiescent stem cells in various tissues, including the skin, mammary gland, and prostate, and its expression is often maintained during early stages of epithelial differentiation. In pathological contexts, CK15 is utilized as an immunohistochemical marker to distinguish basal cell carcinomas from other cutaneous neoplasms and to identify subsets of aggressive breast and prostate cancers. Its role in maintaining stem cell properties and tissue homeostasis makes it a valuable tool in both developmental biology and diagnostic pathology.