The present provides nuclear localization signaling (NLS) sequences derived from titin, comprised of amino acids 181-220: SVGRATSTAE LLVQGEEEVP AKKTKTIVST AQISESRQTR and fragments thereof, such as amino acids 193-208: VQGEEEVP AKKTKTIV; amino acids 199-208: VPAKKTKTIV; and amino acids 200-206: PAKKTKT. The NLS sequences can be linked to agents, such as peptides, proteins or nucleotides, for transporting the agents into the nucleus of cells, and the NLS-agent complex can be further linked to antibodies or ligands for specific binding to cells. Also provided is a method for constructing cDNAs comprising combining a NLS sequence with a nucleic acid sequence for a target protein for expression and entry of the target protein into the nucleus of cells, which then can perform specific functions therein.