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Double-immunostaining of the Wilms' tumor protein Wt1 (purple) and the proliferation marker Ki-67 (brown) in human retinoblastoma. Nuclei were counterstained with hematoxylin (blue). Wt1 immunoreactivity is mainly detected in tumor regions with low expression of Ki-67. Inhibition of Wt1 with antisense oligonucleotides dramatically reduced the capacity of cultured Y-79 retinoblastoma cells to differentiate into neuron-like cells. Thus, Wt1 expression in retinoblastoma may reflect the potential of these tumors to initiate the early steps of neuronal differentiation. For details, see the article by Wagner et al., in this issue.