We note that a paper by Gonza ´lez-Ca ´mpora et al. 1 showed that strong dipeptidyl aminopeptidase IV (DAP IV) expression in an encapsulated follicular lesion is highly suggestive of malignancy and the DAP IV method is a reliable indicator of benign disease if negative results are obtained. We wish to discuss the expression of DAP IV (CD26) in thyroid tissues, both normal and diseased. We also evaluated CD26 as a malignancy marker in thyroid neoplasia. 2 One hundred fifty-five thyroids, normal (fetus, child, adult, and elderly) and diseased, neoplastic and nonneoplastic, were evaluated. Anti-DAP IV IgG 44-4 monoclonal antibody (1:25 dilution; Miyazaki Medical College, Miyazaki, Japan) was used with a sensitive avidin-biotin immunoperoxidase technique. The staining was also performed by a modification of Lojda's method in smears from 74 cases: papillary carcinoma, follicular carcinoma, 51 of colloid goiter, 13 of Hashimoto's thyroiditis, 4 follicular adenoma, oncocytic adenoma, medullary carcinoma, metastasis of renal-cell carcinoma, and diffuse hyperplasia.
DAP IV expression, as related to age, was not found in follicle cells in normal tissues in fetuses or children and tended to express itself in adults after the fifth decade; it was expressed in half of the diseased, nonneoplastic thyroids and related to oncocytic metaplasia. Positivity was increased in neoplasms, especially well-differentiated malignant ones; however, in our opinion, DAP IV is not adequate as a marker for malignancy in individual cases, because it may be present also in normal and diseased, nonneoplastic and benign neoplastic thyroids. Therefore, the enzyme alone is not sufficient to distinguish thyroid carcinoma from benign tumors. Our results hampered the expectation that DAP IV activity staining might indicate malignant potential and have value in the preoperative detection of thyroid nodular disease.