However, we advise that any pediatric biopsy, obtained from the retroperitoneum and showing both nuclear and cytoplasmic WT1 positivity (antibodies directed against the N-terminus WT1 protein: clone WT6F-H2) along with CD56 immunostaining, should alert the pathologist to include Wilms’ tumor in the differential diagnosis, especially in the absence of other conventional markers (CD99, NB84, TdT). The gene discussed is WT1; the disease is Wilms tumor.