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Cover Loss of Argonaute 2 (Ago2) results in a severe hematological disorder. Shown here is an electron micrograph of Ago2-deficient red blood cells. Many features of Ago2 deficiency are evident, such as hypochromic erythrocytes containing inclusion bodies of precipitated globin (Heinz bodies), as well as anisocytosis. These defects cannot be attributed to the loss of Ago2's Slicer endonuclease activity. However analysis of deficient cells revealed a central role for Ago2 in microRNA biogenesis. Please note that the image was colored red by the authors solely for aesthetic purposes. (For details, see O'Carroll et al., p. 1999.)