Extrathyroidal cancers could potentially be targeted with (131)I, if the Na(+)/I(-) symporter (NIS) were functional. Using immunohistochemical methods we probed 1278 human samples with anti-NIS antibody, including 253 thyroid and 169 breast conventional whole tissue sections (CWTS). Four high density tissue microarrays containing a wide variety of breast lesions, normal tissues, and carcinoma cores were tested. The results of the normal microarray were corroborated in 50 CWTS. Nineteen of 34 normal tissues, including bladder, colon, endometrium, kidney, prostate, and pancreas, expressed NIS. Nineteen of 25 carcinomas demonstrated NIS immunopositivity; 55.7% of 479 carcinoma microarray cores expressed NIS, including prostate (74%), ovary (73%), lung (65%), colon (62.6%), and endometrium (56%). NIS protein was present in 75% benign thyroid lesions, 73% thyroid cancers, 30% normal-appearing, peritumoral breasts, 88% ductal carcinomas in situ, and 76% invasive breast carcinoma CWTS. Comparatively, breast microarray cores had lower immunoreactivity. Plasma membrane immunopositivity was confirmed in thyrocytes, salivary ductal, gastric mucosa, and lactating mammary cells. In other tissues, immunoreactivity was predominantly intracellular, particularly in malignant lesions. Thus, NIS is present in many normal epithelial tissues and is predominantly expressed intracellularly in many carcinomas. Elucidating the regulatory mechanisms that render NIS functional in extrathyroidal carcinomas may make (131)I therapy feasible.