The present invention concerns a novel method for the modification and/or custom-made design of artificial non-ribosomal peptide synthetases (NRPSs) from naturally available NRPSs. The artificial NRPSs are of predetermined length and amino acid composition and sequence. Via fusion of well-defined NRPS units (so-called “exchange units”) in a certain manner, using a specific sequence motif in the linker areas it is possible to construct artificial and/or modified NRPS assembly lines, which have the ability of synthesizing peptides of a desired structure.