Biological molecules with the same charge seem to be attracted to each other despite the physical concept that opposite charges attract. Researchers from Lund University and their colleagues elsewhere have observed this paradoxical phenomenon in arginine-rich cell-penetrating antibiotic peptides. "We were very surprised. These biomolecules have a high electrical charge, and the expectation was therefore that this would make them push each other away," says Mikael Lund. Ultimately, it is "salt screening" that allows the self-aggregating tendency of the arginine residues to overcome the repulsive effect of like charges. The phenomenon might be exploited in enhancing drug delivery, the team suggests.