A compound derived from biosynthetic acetyl and/or propionyl units and other building blocks, in which carbonyl carbons condense with a-carbons analogously to the synthesis of fatty acids. They are often fungal metabolites such as macrolides, which are formed by polyketide synthases (PKSs), modular multienzyme complexes encoded as clusters of open reading frames (ORFs) spanning more than 100kbp of the microbial genome, with each ORF responsible for a different enzyme activity. The cluster for any given PKS recruits the ORFs for those transferases that can form the carbon skeleton and the reductases and dehydratases that transform it into the particular polyketide product.