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RNA silencing processes are guided by small RNAs known as siRNAs and microRNAs (miRNAs) [1–4]. They reside in ribonucleoprotein complexes, which guide the cleavage of complementary mRNAs [3, 4] or affect stability and translation of partial complementary mRNAs [1, 2, 5]. Argonaute (Ago) proteins are at the heart of silencing effector complexes and bind the single-stranded siRNA and miRNA [4, 6]. Our biochemical analysis revealed that Ago2 is present in a pre-miRNA processing complex that is able to transfer the miRNA into a target-mRNA cleaving complex. To gain insight into the function and composition of RNA silencing complexes, we purified Ago1- and Ago2-containing complexes from human cells. Several known Ago1- and/or Ago2-associated proteins including Dicer were identified, but also two novel factors, the putative RNA helicase MOV10, and the RNA recognition motif (RRM)-containing …