In molecular biology, ribosomal s6 kinase (rsk) is a family of protein kinases involved in signal transduction.  There are two subfamilies of rsk, p90, also known as MAPK-activated protein kinase-1 (MAPKAP-K1), and p70, also known as S6-H1 Kinase or simply S6 Kinase.  There are three variants of p90 in humans, rsk 1-3.  Rsks are serine/threonine kinases and are activated by the MAPK/ERK pathway.  There are two known mammalian homologues of S6 Kinase: S6K1 and S6K2.