WT1 encodes a transcription factor involved in kidney development and tumorigenesis. Using representational difference analysis, we identified a new set of WT1 targets, including a homologue of the Drosophila receptor tyrosine kinase regulator, sprouty. Sprouty1 was up-regulated in cell lines expressing wild-type but not mutant WT1. WT1 bound to the endogenous sprouty1 promoter in vivo and directly regulated sprouty1 through an early growth response gene-1 binding site. Expression of Sprouty1 and WT1 overlapped in the developing metanephric mesenchyme, and Sprouty1, like WT1, plays a key role in the early steps of glomerulus formation. Disruption of Sprouty1 expression in embryonic kidney explants by antisense oligonucleotides reduced condensation of the metanephric mesenchyme, leading to a decreased number of glomeruli. In addition, sprouty1 was expressed in the ureteric tree and antisense-treated ureteric trees had cystic lumens. Therefore, sprouty1 represents a physiologically relevant target gene of WT1 during kidney development.The development of the mammalian metanephric kidney is a model for the study of cellular and molecular mechanisms of organogenesis (1). Wilms tumor, a pediatric kidney malignancy, is characterized by a triphasic histopathology (blastemal, stromal, epithelial) signifying an abnormal differentiation program. In accordance with this notion, the Wilms Tumor suppressor gene 1 (WT1), inactivated in a subset of Wilms tumors, plays an essential role in normal development of the kidney and the genitourinary system (2-4). Targeted disruption of WT1 in mice leads to a complete agenesis of the kidneys and gonads (5). The specific temporal and spatial pattern of WT1 expression suggests multiple roles for WT1 during nephrogenesis. In particular, WT1 expression peaks during the mesenchymal-to-epithelial transition (MET), 1 suggesting an instructive role in the formation of the renal glomerulus. In the mature kidney, WT1 expression becomes restricted to the podocytes, perhaps being involved in maintaining a differentiated phenotype.The WT1 gene encodes a C 2 -H 2 zinc finger transcription factor that binds to both GC-rich and TC repeat elements. Alternative splicing at two sites yields four major isoforms, each containing or lacking 17 amino acids between the transactivation domain and the zinc finger region and/or a 3-amino acid insertion (KTS) between the third and fourth zinc fingers. The KTS insertion disrupts the spacing between the zinc fingers, altering DNA binding (2, 4). The A isoform lacks both these motifs and binds strongly to DNA. The C isoform contains the KTS insertion, displays a weaker DNA binding to alternative sequences, associates with splicing factors, and therefore, may be involved in RNA processing. WT1 binding sites were identified in multiple promoters that respond to WT1 in transient transfection assays (2, 6, 7); however, most are not regulated by WT1 in vivo (8). Thus, identification of bona fide target genes would provide insight into WT1 function in development and tumo...