Kallikreins (tissue and plasma kallikreins) are serine proteases that liberate kinins (bradykinin and kallidin) from the kininogens, which are plasma proteins that are converted into vasoactive peptides. The Kallikrein Inhibitor Peptide corresponds to aa386-391 of bovine kininogen-1 that encompasses the aa388-389 kallikrein proteolytic site. The synthetic kallikrein inhibitor can attenuate breast cancer cell invasion, therefore it is investigated for its role in invasion and metastasis of cancer cells.Ac-Pro-Phe-Arg-Ser-Val-Gln-NH2 or Ac-PFRSVQ-NH2