Immunohistochemical (IHC) testing reveals the nuclear expression of the transcription factor WT-1 (Wilms' Tumor 1). WT-1 protein is normally expressed during embryonic development in kidney and gonadal tissues. In diagnostic pathology, it is a highly useful marker for the diagnosis of high-grade serous ovarian cancer, primary fallopian tube cancer, and primary peritoneal cancer. It is also a key marker in the diagnosis of Wilms' tumor (nephroblastoma) in children and in the evaluation of certain soft tissue tumors.