The MAPKAP kinases are a group of MAP kinase substrates that are themselves kinases. In response to activation, the MAP kinases phosphorylate downstream components on a consensus Pro-X-Ser/Thr-Pro motif. Several kinases that contain this motif have been identified and serve as substrates for the ERK and p38 MAP kinases. Mitogen-activated protein (MAP) kinase-activated protein kinase 3, also known as MAPKAPK-3 and 3pK, is a member of the Ser/Thr protein kinase family. It is widely expressed in human tissues, with a higher expression level observed in the heart and skeletal muscle. No expression in the brain. MAPKAPK-3 is unique since it was shown to be activated by three members of the MAPK family, namely extracellular-signal-regulated kinase (ERK), p38, and Jun-N-terminal kinase (JNK). It is highly activated both by mitogens and by stress-inducing agents or proinflammatory cytokines and translocates to the cytoplasm from the nucleus. MAPKAPK-3 is exclusively activated via the classical MAPK cascade, while stress-induced activation of MAPKAPK-3 is mainly mediated by p38, however, the mechanism defining the specificity remains unknown.