The stability, size, and structure of avian pancreatic polypeptide (aPP) makes it a useful starting point for the design of miniature proteins that bind with high affinity and specificity to DNA1-4 and proteins5-14 and inhibit their interactions, both with high affinity and specificity, both in vitro1-6,8,9,11-14 and in mammalian cells and extracts.7,10 The utility of these molecules in a cellu...