The tumor suppressor WT1 represses and activates transcription. The loss and/or imbalance of the dual transcriptional activity of WT1 may contribute to Wilms tumor. Par-4 (for prostate apoptosis response) is a WT1-interacting protein that also functions as a transcriptional repressor. Par-4 contains a putative leucine zipper domain and is specifically upregulated during apoptosis of prostate cells.