WT1 and Nephroblastoma: 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).