Mitogen- and stress-activated protein kinase 1 (MSK1), also known as ribosomal protein S6 kinase 5 (RPS6KA5), belongs to the AGC family of kinases. Both MSK1 and the related MSK2, also known as RSKB and RPS6KA4, have two kinase domains connected by a regulatory linker region, and are activated by the mitogen-activated protein kinases ERK1, ERK2, and p38. Nuclear MSK phosphorylates and activates a number of transcription factors, including ATF1 and CREB.