Molecular transporters, which deliver attached cargo such as drugs or proteins into cells, commonly feature positively charged arginine or other guanidinium-containing components. In a new report, Alanna Schepartz and Douglas S. Daniels at Yale University describe novel types of molecular transporters made from arginine and proline units that outperform some arginine-only transporters (J. Am. Chem. Soc. 2007, 129, 14578). The proline imparts secondary structure that the arginine-only transporters don't have. The team's best transporters contain five or six repeating PRR units, where P is proline and R is arginine. The five-unit transporter matches the performance of the corresponding arginine-only transporter with 10 arginines, and the six-unit transporter outperforms the equivalent 12-arginine transporter. The researchers incorporated the transporters into a small protein in order to successfully deliver it into cells.