Cover photograph (Copyright © 2008, American Society for Microbiology. All Rights Reserved.): Ion channels capable of passing superoxide from the interior to the exterior of endosomes exist and likely play a role in redox signaling by the interleukin-1β (IL-1β) receptor. IL-1β-stimulated NADPH oxidase leads to the production of superoxide inside early endosomes, as shown in the center cell loaded with H2HFF-bovine serum albumin, which fluoresces green when it reacts with superoxide; the nucleus is labeled blue with DAPI (4′,6′-diamidino-2-phenylindole). The inset shows a transmission electron micrograph of a Rab5 endosome (pseudocolored red) adhered to a magnetic bead. Large gold particles mark the IL-1 receptor, while small gold particles mark superoxide dismutase 1 (SOD1) recruited to the receptor. These findings suggest that SOD1 may play an active role in converting superoxide to hydrogen peroxide to facilitate redox-dependent receptor signaling. (See related article on p. 3700.)