WT1 and Wilms tumor: A histopathologic specimen of the kidney mass from Wilms tumor would show undifferentiated cells of small to medium size with apparently normal-looking nuclei having minute-sized nucleoli (Figure 13). These indicate that the tumor occurs in primitive or primitive-like cells. Immunohistochemical analysis is very important here, and it is worth noting that the WT1 marker was strongly positively expressed (Figure 14). The high level of WT1 expression is in line with the characteristic pathological findings of Wilms tumor, and it represents a useful tool to identify this childhood renal neoplasm.