A pharmaceutically acceptable composition and method for entering a cell nucleus utilizes a cell nucleus-entering polypeptide including at least one of amino acid sequence LKKTET (SEQ ID NO:1), amino acid sequence LKKTNT (SEQ ID NO:2) or amino acid sequence KSKLKK (SEQ ID NO:3), or a conservative variant thereof, linked to a physiologically active agent having at least one of therapeutic or diagnostic application in the cell nucleus.