MSK1, a Mitogen and Stress activated protein Kinase, is activated by ERK as well as p38 MAPK in response to growth factors and various cellular stress stimuli. MSK1 resembles RSK by having two kinase domains connected by a regulator linker region. Phosphorylation of RSK1 at Ser 364, Ser381 and Thr574 is critical for RSK1 activity. These sites are analogous to Ser360, Ser376 and Thr581 of MSK1 and maybe important for MSK1 activity as well.