speculate that the intracellular degradation of SOX17 is at least partially dependent on tyrosine-phosphorylation at residue 259. SOX17 is known to directly interact with transcriptional effectors of the Wnt signaling pathway, b-catenin, and TCF/LEF. SOX17 physically interacts with b-catenin to regulate SOX17 target genes [Sinner et al., 2004]. In addition, SOX17 is a potent Wnt-signaling antagonist, and does so through direct physical interaction with both b-catenin and TCF/LEF factors [Sinner et al., 2007]. Formation of this trimeric complex leads to the degradation of both b-catenin and Tcf/Lef proteins through a GSK3-independent mechanism and loss of Wnt-signaling activity. Consistent with SOX17 acting to inhibit the Wnt/b-catenin pathway, our data showed that elevated SOX17 p.Y259N protein inappropriately suppressed the Wnt signaling pathway in renal cells. Because mice lacking b-catenin in the developing kidney have symptoms of CAKUT, it is possible that altered levels of Wnt signaling during development could cause the observed congenital defects in these patients with CAKUT. Alternatively, elevated levels of Sox17 could effect urinary tract development through regulation of Sox17 target genes, of which several have been identified [Patterson et al., 2008; Sinner et al., 2004]. SOX17 is expressed throughout different stages of kidney development (http://www.gudmap.org) [Brunskill et al., 2008]. For example, in situ hybridization and microarray analysis shows that Sox17 is highly expressed in bladder stroma and to a minor degree in the ureteric bud and in metanephric mesenchyme at E11.5 days after fertilization, a stage where Wnt9b acts to regulate ureteric branching [Carroll et al., 2005]. Other Wnt ligands, specifically Wnt2b, Wnt6, Wnt7b, and Wnt9b, as well as Sox17, are also expressed at later stages of kidney development such as in the S-shaped bodies, mesonephros, and metanephros, and later in the medullary collecting duct, renal medullary interstitium, maturing renal corpuscle, early proximal tubule, and cortical collecting duct. Our in vitro analysis of SOX17 activity showed a clear correlation between SOX17 protein level and the degree of b-catenin repression, demonstrating a dose-dependent inhibitory effect of SOX17 on Wnt signaling. This is interesting because the proband’s duplicated chromosome 8 contained two copies of the SOX17– p.Y259N mutation and would be predicted to exhibit a genetic ‘‘dose-dependent’’ increase in phenotypic severity. In fact, the patient with the duplication and two copies of SOX 17– p.Y259N, which would be predicted to have higher accumula- tion of SOX17 and more suppression of Wnt/b-catenin, had more severe developmental defects than the patients with one copy of SOX 17– p.Y259N. To our knowledge, no mutation or disease has been associated with the SOX17 gene in humans. Only two HMG box mutations were found in cell lines from colorectal cancers [Suraweera et al., 2006]. The presence of SOX17 mutations in several patients with urinary tract anomalies suggests that SOX17 is involved in regulatory and signaling pathways controlling the normal development of the urinary apparatus. The additional pathologi- cal features found exclusively in the proband of Family 1, such as psychomotor and language development delay and abnormal corpus callosum, were very likely due to the presence of the 13-Mb duplication spanning a portion of chromosome 8 containing over 30 genes. The reason for the rare association of SOX17 functional mutations with diseases in humans probably resides on the fact that to date, over 20 SOX genes have been identified in vertebrates with closely related family members. For example, SOX17, SOX7, and SOX18, share high structural and functional similarities and have functional redundancies during development [Bowles et al., 2000; Sakamoto et al., 2007; Travers, 2000]. This indicates that Sox 1358 HUMAN MUTATION, Vol. 31, No. 12, 1352–1359, 2010 family members that are coexpressed in the same cell type will functionally substitute for one another. In conclusion, the identification of SOX17 mutations in individuals with duplication of the urinary tract and VUR has broad implications for the genetic workup in patients with kidney and urinary tract malformations. SOX17 is the latest in a series of genes (PAX2, ROBO2) whose alteration induces partial defects of the VUR. 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