Recent studies in our lab identified a mutant mouse model of obstructive nephropathy designated mgb for megabladder. Homozygotic mgb mice (mgb−/−) develop lower urinary tract obstruction in utero due to a lack of bladder smooth muscle differentiation. This defect is the result of a random transgene insertion/translocation into chromosomes 11 and 16. Transcriptional profiling identified a significantly over‐expressed cluster of gene products located on the translocated fragment of chromosome 16 including urotensin II‐related peptide (Urp), which was shown to be preferentially over‐expressed in developing mgb−/− bladders. Pathway analysis of mgb microarray data indicated dysregulation of at least 60 gene products associated with smooth muscle development. In conclusion, the results of this study indicate that the molecular pathways controlling normal smooth muscle development are severely altered in mgb−/− bladders, and provide the first evidence that Urp may play a critical role in bladder smooth muscle development. Developmental Dynamics 237:170–186, 2008. © 2007 Wiley‐Liss, Inc.