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MicroRNAs (miRNAs) represent a family of small noncoding RNAs that are found in plants and animals (for recent reviews, see [1–5]). miRNAs are expressed in a developmentally and tissue-specific manner and regulate the translational efficiency and stability of partial or fully sequence-complementary mRNAs. miRNAs are excised in a stepwise process from double-stranded RNA precursors that are embedded in long RNA polymerase II primary transcripts (pri-miRNA) [6–10]. Drosha RNase III catalyzes the first excision event, the release in the nucleus [11–13] of a hairpin RNA (pre-miRNA), which is followed by export of the pre-miRNA to the cytoplasm [14–16] and further processing by Dicer to mature miRNAs [17–22]. Here, we characterize the human DGCR8, the DiGeorge syndrome critical region gene 8, and its Drosophila melanogaster homolog. We provide biochemical and cell-based readouts to …