Ribosomal protein S6 is the major substrate of protein kinases in eukaryote ribosomes. The phosphorylation is stimulated by growth factors, tumor promoting agents and mitogens and the dephosphorylation by growth arrest. The 229-239 peptide contain two phosphorylation sites, Ser235 and Ser236.Human: H-Ala-Lys-Arg-Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala-OH or H-AKRRRLSSLRA-OH