S6-1 Peptide is the major substrate of protein kinasses in eukaryote ribosomes. The phosphorylation is stimulated by growth factors , tumor promoting agents, and mitogens. It is dephosphorylated at growth arrest.Human: H-Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala-OH or H-RRLSSLRA-OH