Protein Domain Fold Similarity and Natural Product Structure as Guiding Principles for Compound Library Design -- Sources of Polyketides and Non-Ribosomal Peptides -- Polyketide Synthases: Mechanisms and Models -- Functional and Structural Basis for Targeted Modification of Non-Ribosomal Peptlde Synthetases -- Prerequisites for Combinatorial Biosynthesis: Evolution of Hybrid NRPS/PKS Gene Clust…