Obstructive nephropathy: Insights from genetically engineered animals. Congenital obstructive nephropathy is the primary cause for end-stage renal disease (ESRD) in children. An increasingly used animal model of obstructive nephropathy is unilateral ureteral obstruction (UUO). This model mimics, in an accelerated manner, the different stages of obstructive nephropathy leading to tubulointerstitial fibrosis: cellular infiltration, tubular proliferation and apoptosis, epithelial-mesenchymal transition (EMT), (myo)fibroblast accumulation, increased extracellular matrix (ECM) deposition, and tubular atrophy. During the last decade genetically modified animals are increasingly used to study the development of obstructive nephropathy. Although the use of these animals (mainly knockouts) has highlighted some pitfalls of this approach (compensation by closely related gene products, absence of temporal knockouts) it has brought important information about the role of specific gene-products in the pathogenesis of obstructive nephropathy. Besides confirming the important pathologic role for angiotensin II (Ang II) and transforming growth factor-β (TGF-β) in obstructive nephropathy, these animals have shown the complexity of the development of tubulointerstitial fibrosis involving a large number of closely functionally related molecules. More interestingly, the use of these animals has led to the discovery of unexpected and contradictory roles (both potentially pro- and antifibrotic) for Ang II, for ECM degrading enzymes matrix metalloproteinase 9 (MMP-9) and tissue plasminogen activators (PAs), for plasminogen activator inhibitor 1 (PAI-1), and for the adhesion molecule osteopontin (OPN) in obstructive nephropathy. Further use of these animals, especially in combination with pharmacologic tools, should help to better identify potential antifibrotic strategies in obstructive nephropathy.