p70 S6 kinase is a mitogen activated Ser/Thr protein kinase that is required for cell growth and G1 cell cycle progression. P70 S6 kinase phosphorylates the S6 protein of the 40S ribosomal subunit and is involved in translational control of 5’oligopyrimidine tract mRNAs. A second isoform, p85 S6 kinase, is derived from the same gene and is identical to p70 S6 kinase except for 23 extra residues at the N-terminus that encode a nuclear localizing signal. Both isoforms lie on a mitogen-activated signaling pathway downstream of phosphoinositide 3 kinase (PI3K) and the target of rapamycin FRAP/mTOR, and distinct from the Ras/MAP kinase cascade.