p70S6K is responsible for the phosphorylation of 40S ribosomal protein S6 and is ubiquitously expressed in human adult tissues . p70S6K is activated by serum stimulation and this activation is inhibited by wortmannin and rapamycin. p70S6k activity undergoes changes in the cell cycle and increases 20-fold in G1 cells released from G0 . p70S6K activation requires sequential phosphorylations at proline-directed residues in the putative autoinhibitory pseudosubstrate domain, as well as threonine 389 a site phosphorylated by phosphoinositide-dependent kinase 1 (PDK-1).