PROBLEM TO BE SOLVED: To provide a cell membrane permeable boron formulation permeating a cell membrane and localized in a cell, especially in a nucleus.SOLUTION: The cell membrane permeable boron peptide includes a cell membrane permeable peptide and a boron compound. The cell membrane permeable peptide has an amino acid sequence of, for example, consecutive 9-13 residues of TAT or arginine, and the boron compound may be, for example, mercapto undecahydrododecaborate. The cell membrane permeable boron peptide including the cell membrane permeable peptide and the boron compound can be combined with at least one pharmacologically acceptable excipient to produce the boron formulation.SELECTED DRAWING: None