On the cover: Receptor activation and desensitization both transmit G protein-coupled receptor (GPCR) signals. A, Ligand (H) binding to a GPCR catalyzes heterotrimeric G protein activation and a "first wave" of signal transduction. B, GPCR desensitization switches the receptor into distinct signaling modes that generate a "second wave" of signaling. (See page 177.)