The present invention is to provide a polypeptide specifically inhibiting the activity of Akt (Protein Kinase B), the DNA thereof, the antibody thereof, an inhibitor of Akt activity or an antitumor agent, and the like. The polypeptide comprises polypeptides (SEQ IDNO: 1, 3, 5, 7, and 9 of the sequence listing) that contain an amino acid sequence corresponding to any of the position of amino acid residue10-24 of human TCL1, amino acid residue8-22 of human TCL1B, amino acid residue5-19 of human MTCP1, and amino acid residue9-24 of mouse or rat TCL1 and the derivatives. Further, the present invention includes DNA encording the polypeptide (SEQ ID NO: 2, 4, 6, 8 or 10 of the sequence listing), and the antibodies specifically binding to the polypeptides. The polypeptide of the present invention can be used for an inhibitor of Akt activity, an antitumor agent, or the like.
