A) During UB epithelial development, the let-7 family of miRNAs exhibits a temporally regulated gene expression pattern which is inversely correlated with the temporal expression pattern of putative early UB let-7 target genes. This temporal expression pattern of the let-7 family miRNAs and their targets is altered in the Dicer mutant UB epithelium, where the expression of the let-7 family miRNAs is disrupted, and the expression of early let-7 targets persists in the later stages of UB epithelial morphogenesis. B) During normal kidney development, the expression of early UB genes at early developmental stages defines the undifferentiated UB epithelium and inhibits the expression of late genes (collecting duct differentiation genes). At later developmental stages, the let-7 family miRNAs, directly or through their target genes, inhibit the expression of some of the early UB genes, thereby facilitating the expression of late genes in the UB epithelium, and this late gene expression defines the collecting duct fate.