Proteins are the most important molecules inside our body. There are thousands of proteins in a single cell alone and they control our physiological reactions, metabolism, cellular information flow, defense mechanisms - pretty much everything. No wonder then that most human diseases are related to the malfunctioning of particular proteins. In contrast to gene therapy - where a gene is placed inside a cell to either replace a defective gene or to increase the amount of a specific gene in order to produce a higher amount of a desired protein - protein therapy works by directly delivering well-defined and precisely structured proteins into the cell to replace the dysfunctional protein. The problem with protein therapy, which limits its practical use in medicine, is the mode of delivery. Scientists have now demonstrated a general, effective, low-toxicity intracellular protein delivery system based on single-protein nanocapsules.