Ectopic budding of the initial ureter from the Wolffian duct is the first ontogenic misstep that leads to many congenital anomalies of the kidney and urinary tract (CAKUT). The ectopia results in hypoplastic kidney, ectopia of ureterovesical orifice, urinary outflow obstruction and/or reflux. Recent studies on several mutant mouse models verified that ectopic ureteric budding indeed occurs prior to the formation of CAKUT. Often, the genes involved in navigating the site of ureteric budding also regulate later ontogenic processes of the kidney and other urinary tract systems. These additional functions of the genes underlie the wide spectrum of CAKUT, for example multicystic dysplastic kidney, megaureter and atretic ureter, because the genes regulate the morphogenesis of the many portions of the excretory system through their distinctive cellular functions.