p90 RSK, also known as RSK1, is a member of the 90 kDa ribosomal S6 kinase family, which includes RSK1, RSK2, and RSK3. These are serine/threonine protein kinases activated in response to mitogenic stimuli, including extracellular signal-regulated protein kinases Erk1 and Erk2. RSK1 is activated by MAPK in vitro and in vivo via phosphorylation. Active RSKs play a major role in transcriptional regulation by translocating to the nucleus and phosphorylating c-Fos and CREB, which are key regulators of gene expression. The phosphorylation of RSK1 at T359/S363 is part of the signaling cascade initiated by growth factors and hormones, which leads to cellular proliferation and survival.