“…The DH domain generates a 2-enoyl thioester intermediate via syn -elimination of a 3-hydroxy group, resulting in the production of alkenes in even-to-odd positions in the final polyketide structure. − DHs are notable for a strict stereospecificity of the 3-hydroxy substrate, which has been correlated with the geometry of the product double bond. − , Despite high-resolution crystal structures of several type-I PKS DHs, the structural basis for substrate specificity and product selectivity remains unclear. ,− While “A” and “B” sequence motifs within KR domains predict the stereochemical configuration of the 3-hydroxy group, no such motifs have been identified in PKS DHs. , To date, only DHs that act on A-type KR products have been shown to produce cis double bonds, whereas B-type KR reduction followed by DH dehydration leads to trans double bonds in most cases, ,,, with notable exceptions. ,,, …”