Several proteins can traverse biological membranes through protein transduction. Small sections of these proteins (10-16 residues long) are responsible for this. Linking these domains covalently to compounds, peptides, antisense peptide nucleic acids or 40-nm iron beads, or as in-frame fusions with full-length proteins, lets them enter any cell type in a receptor- and transporter-independent fashion. Moreover, several of these fusions, introduced into mice, were delivered to all tissues, even crossing the blood-brain barrier. These domains thus might let us address new questions and even help in the treatment of human disease.