Due to the similarity with polyketide synthases (PKS), many secondary metabolites are, in fact, fusions of NRPs and polyketides.  In essence, this occurs when PK modules follow NRP modules, and vice versa.  Although there is high degree of similarity between the Carrier (PCP/ACP) domains of both types of synthetases, the mechanism of condensation is different from a chemical standpoint:
* PKS, carbon-carbon bond formation through Claisen condensation reaction
* NRPs, the C domain catalyzes the amide bond formation between the amino acid it adds to the chain (on the PCP of one module) and the nascent peptide(on the PCP of the next module).