The present invention relates to production of polyketides and other natural products and to libraries of compounds and individual novel compounds. One important area is the isolation and potential use of novel FKBP-ligand analogues and host cells that produce these compounds. The invention is particularly concerned with methods for the efficient transformation of strains that produce FKBP analogues and recombinant cells in which cloned genes or gene cassettes are expressed to generate novel compounds such as polyketide (especially rapamycin) FKBP-ligand analogues, and to processes for their,preparation, and to means employed therein (e.g. nucleic acids, vectors, gene cassettes and genetically modified strains).