Kidney-filtered waste products are excreted from the body via the urinary tract, which comprises the intra-renal collecting system and the ureter. These structurally and functionally distinct tissues derive from the ureteric bud(UB). Now Doris Herzlinger and co-workers report that two mesenchymal cell populations - the nephrogenic mesenchyme and the tailbud-derived mesoderm(TBM) - surround the UB, and that the ureter develops as a consequence of the distal UB's close association specifically with the TBM(p. 1967). Their fate-mapping studies in the chick show that BMP4 secreted from the TBM induces ureter morphogenesis and can do so when expressed ectopically in regions of the UB normally fated to develop into the intra-renal collecting system,revealing the multipotent nature of the proximal UB. The authors suggest that the complex morphogenetic processes required to bring the TBM into close contact with the UB might contribute to the high incidence of human congenital defects that occur at this junction between the kidney and the ureter.