Renal dysplasia/hypoplasia is a leading cause of renal failure in children, leading to significant morbidity and mortality associated with transplan and dialysis. The risk of chronic kidney disease is linked to decreased renal reserve as a result of the formation of fewer and/or abnormal nephrons during kidney development. While much is known about the genetic control of nephron development, very little is known about the role of microRNAs (miRNAs), small, non-coding RNA molecules that negatively regulate gene expression. Our laboratory has data demonstrating that the miR-17~92 miRNA cluster is crucial to regulating nephron number and formation. Conditional loss of miR-17~92 in nephron progenitors results in renal hypodysplasia, glomerular injury and renal dysfunction in adult mice. Moreover, we observe an intermediate phenotype in animals with heterozygous loss of miR-17~92 in nephron progenitors, suggesting that the gene dosage of miR- 17~92 is key. Heterozygous mutations in the orthologous human gene (MIR17HG) results in the first known developmental defects associated with a miRNA mutation in humans, including renal anomalies. We hypothesize that loss of the miR-17~92 cluster in nephron progenitors results in an intrinsic nephron progenitor defect, and therefore abnormal nephron number and pattern during kidney development. Aim 1. Define the role of miR-17~92 gene dosage in establishing nephron number and pattern. Aim 2. Characterize the intrinsic defect in miR-17~92 null nephron progenitors. Aim 3. Validate downstream miR-17~92 targets to elucidate mechanism(s) by which the miR-17~92 cluster regulates nephron number and patterning.