In a modular PKS such as the 6-deoxyerythronolide B synthase, sets of catalytic domains are organized into modules. The catalytic domains from a given module cooperate to synthesize polyketides via a thiotemplate mechanism similar to that of fatty acid synthesis. The nascent polyketide intermediates are then passed down to the next module in the protein assembly line. Organized into three polypeptides (DEBS1-3), DEBS consists of six catalytic modules, each containing a unique set of covalently linked catalytic domains. Together, the six modules of this megasynthase (approximated 2MDa) produce 6-deoxyerythronolide B (6-dEB), the polyketide portion of the antibiotic erythromycin.