RSK3(Ribosomal S6 kinase 3)is a member of a family of 90-kDa ribosomal protein S6 kinases, which includes Rsk1, Rsk2 and Rsk3. These are broadly expressed serine/threonine protein kinases that are activated in response to mitogenic stimuli, including extracellular signal-regulated protein kinases Erk1 and Erk2. Rsk3 is a distinct isoform of p90Rsk that translocates to the cell nucleus, phosphorylates potential nuclear targets and may have a unique upstream activator. Several sites, such as Ser380, Thr359, Ser363 and Thr573, are important for its activation. Active Rsks appear to play a major role in transcriptional regulation by translocating to the nucleus and phosphorylating c Fos and CREB.