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MicroRNAs (miRNAs) are small non-coding RNAs that post-transcriptionally regulate gene 25 expression. Hundreds of miRNAs are expressed in mammals, however their functions are just starting to be uncovered. miRNAs are processed from a long hairpin mRNA transcript, down to a ~23 nucleotide duplex. The enzyme Dicer1 is required for miRNA processing and mouse knockouts of Dicer1 are embryonic lethal before E7.5. To examine the function of miRNAs specifically in the germline we used a mouse model which expresses Cre recombinase from the TNAP locus and a floxed 30 Dicer1 conditional allele. Removal of Dicer1 from germ cells resulted in male infertility. Germ cells were present in adult testes, but few tubules contained elongating spermatids. Germ cells that did differentiate to elongating spermatids exhibited abnormal morphology and motility. Rarely, sperm lacking Dicer1 could fertilize wild type eggs to generate viable offspring. These results show that Dicer1 and miRNAs are essential for proper differentiation of the male germline. 3