The NPF peptide encompasses the tyrosine-1355 (Y1355) phosphorylation site within the insulin receptor. This is an autocatalytic site for insulin receptor tyrosine-protein kinase activity.Human: H-Lys-Arg-Ser-Tyr(PO3H2)-Glu-Glu-His-Ile-Pro-OH or H-KRSY(PO3H2)EEHIP-OH