The Wilms' tumor 1 (WT1) gene is originally identified in Wilms tumor, a childhood kidney cancer. This gene encodes a zinc finger transcriptional regulatory protein that has been implicated in growth, normal development, survival and apoptosis. Alternative splicing of the WT1 transcript generates four major protein isoforms and thirty six minor protein isoforms, each having different functional properties. The expression and role of WT1 also varies by cell type and its interacting proteins. Although the WT1 gene has been considered as a tumor suppressor gene, a wild type WT1 gene is expressed in several cancers 436 including leukemia, breast and colon cancer. Since the mechanism of WT1 in breast and other cancers remains unclear, this review focuses on functional activity as a transcription factor, its role in growth and proliferation, and as a tumor suppressor gene and oncogene.