WT1 is a gene involved in the induction of Wilms’ tumor. In normal human tissues, WT1 mRNA has been observed in kidney, spleen and gonadal ridge mesoderm (1). WT1 expression has also been observed in sertoli cells of testes and in granulosa cells of the ovary. In tumors, WT1 has been demonstrated in Wilms’ tumors and in the majority of mesotheliomas (nuclear and paranuclear staining) (2). WT1 has also been demonstrated in the majority of acute leukemias, but not in cells from chronic myelogenous leukemia (3). Cytoplasmic staining has been observed in some cases of adenocarcinoma and may represent crossreactivity with an epitope unrelated to WT1 (2).