A deeper understanding of molecular interactions in binding sites can help guide the rational design of inhibitors or custom-building blocks which, if tolerated and processed by the rest of the assembly line, can help generate new bioactive peptides. The first crystal structure of an NRPS adenylation domain was solved in 1997, and the impact of the detailed analysis of PheA is still recognized to this day. We have chosen to highlight all NRPS structures deposited into the Protein Data Bank (PDB) that contain an A domain (Table ), many of which are multi-domain proteins or entire modules, and a small subset of these structures have been summarized previously. These protein structures have provided strong evidence for the activation of a wide variety of building blocks (Figure ), including putative residues involved in direct substrate interactions or else contributing to the size, shape and chemistry of the binding pocket.