Many important natural products such as antibiotics, immunosuppressants and cancer drugs are derived from microorganisms. These natural products are often small proteins or peptides generated in the cell by NRPS enzymes similar to a modern automobile factory: At each station, additional parts are added to the basic structure until finally, a completed automobile leaves the factory. With regard to the NRPS, a specific amino acid is incorporated and processed at each station (module), and complete peptides emerge that can be linear, cyclic or otherwise modified, including unusual amino acids.