Insulin-degrading enzyme (IDE, insulysin, insulinase; EC 220.127.116.11), a thiol metalloendopeptidase, is involved in intracellular degradation of insulin, thereby inhibiting its translocation and accumulation to the nucleus. Recently, protein expression of IDE has been demonstrated in the epithelial ducts of normal breast and breast cancer tissue. Utilizing four different antibodies generated against different epitopes of the IDE molecule, we performed Western blot analysis and immunohistochemical staining on several normal human tissues, on a plethora of tumor cell lines of different tissue origin, and on malignant breast and ovarian tissue. Applying the four IDE-directed antibodies, we demonstrated IDE expression at the protein level, by means of immunoblotting and immunocytochemistry, in each of the tumor cell lines analyzed. Insulin-degrading enzyme protein expression was found in normal tissues of the kidney, liver, lung, brain, breast and skeletal muscle, as well as in breast and ovarian cancer tissues. Immunohistochemical visualization of IDE indicated cytoplasmic localization of IDE in each of the cell lines and tissues assessed. In conclusion, we performed for the first time a wide-ranging survey on IDE protein expression in normal and malignant tissues and cells thus extending our knowledge on the cellular and tissue distribution of IDE, an enzyme which to date has mainly been studied in connection with Alzheimer's disease and diabetes but not in cancer.