Abstract: A monoclonal antibody, 3E10, and active fragments thereof that selectively are transported in vivo to the nucleus of mammalian cells without cytotoxic effect are provided. The antibody and other molecules that bind to a variant of myosin IIb heavy chain found in the nucleus of skeletal muscle cells are useful as a non-viral delivery vector to target skeletal muscle in vivo. By contrast, in vitro the monoclonal antibody penetrates and is transported to the nucleus of multiple cell lines derived from different tissue types and can be used in screening tests to identify molecules that modulate growth of cells, such as cancer cells. Non-cytotoxic vectors for delivering a drug, polynucleotide or polypeptide selectively to skeletal muscle cells are also provided.