The Wilms' tumour suppressor gene WT1 is essential for the normal development of the genitourinary system. It appears to play a role in both transcriptional and post-transcriptional regulation of certain cellular genes. However, the mechanisms behind WT1 function are not clearly understood despite the identification of numerous potential target genes and the isolation of several WT1-binding proteins. This study therefore sets out to identify other WT1-associating proteins to help to unravel how WT1 interacts with the cellular machinery. We report the identification of a novel human WT1-associating protein, WTAP, which was isolated using the yeast two-hybrid system. Both in vitro and in vivo assays have shown that the interaction between WTAP and WT1 is specific and occurs endogenously in cells. The mouse homologue of WTAP was isolated and found to be >90% conserved at the nucleotide and protein levels. The human and mouse genes were mapped using fluorescence in situ hybridization to regions in chromosomes 6 (which is thought to harbour a tumour suppressor gene) and 17, respectively. The expression pattern of WTAP was investigated and shown to be ubiquitous, perhaps reflecting a housekeeping role. WTAP is a nuclear protein, which like WT1 localizes throughout the nucleoplasm as well as in speckles and partially co-localizes with splicing factors. Although the significance of this interaction is not yet known, WTAP promises to be an interesting WT1-binding partner.