p70 S6 kinase and p90 Rsk kinase are the two distinct 40S ribosomal protein S6 Ser/Thr kinases present in somatic animal cells. p90 Rsk kinase is maximally activated within minutes of addition of growth factors or phorbol ester to cultured cells followed by activation of p70 S6 kinase. Both enzymes are regulated by serine/threonine phosphorylation, sμggesting that specific kinases may exist upstream in the signaling pathways that regulate these kinases.