PAX-8 is a member of the paired box (PAX) family of transcription factors, which are key regulators in early development. This protein plays a role in development of thyroid follicular cells and the expression of thyroid-specific genes, with mutations in the PAX-8 gene linked to thyroid follicular carcinomas, atypical thyroid adenomas, and thyroid dysgenesis. The PAX-8 protein is expressed in simple ovarian inclusion cysts and non-ciliated mucosal cells of the fallopian tubes, but is absent from normal ovarian surface epithelial cells. PAX-8 is also not expressed in normal lung or lung carcinomas. Reports have associated PAX-8 expression with renal carcinoma, nephroblastoma, and seminoma, and have indicated PAX-8 as a useful marker for renal epithelial tumors, ovarian cancer, and for differential diagnoses in lung and neck tumors. Anti-PAX-8 can be useful in determining the primary site of invasive micropapillary carcinomas of ovary from bladder, lung, and breast, when used in adjunct with a panel of organ-specific markers such as uroplakin, mammaglobin, and TTF-1.