Peptides and their derivatives are a rapidly expanding class of therapeutic molecules, in part because they condense the expansive recognition properties of a protein into a more compact molecular framework. The extended binding surface of a peptide provides a better complement to the extended and featureless surfaces that comprise many traditionally "undruggable" targets . Perhaps the greatest attribute of peptides, whether linear or cyclic, is that they are genetically encodable, and hence their properties can be optimized using directed evolution methods performed in vitro or in vivo. Many of these methods depend on ribosomal translation and reliably return peptide leads for challenging and clinically relevant targets. For these reasons, de novo peptides have attracted significant attention as the next frontier in pharmaceutical development .