The function of HACD1 (3-hydroxyacyl-CoA dehydratase 1, Ensembl gene identifier ENSG00000165996) is as follows. [Isoform 1]: Catalyzes the third of the four reactions of the long-chain fatty acids elongation cycle. This endoplasmic reticulum- bound enzymatic process, allows the addition of two carbons to the chain of long- and very long-chain fatty acids/VLCFAs per cycle. This enzyme catalyzes the dehydration of the 3-hydroxyacyl-CoA intermediate into trans-2,3-enoyl-CoA, within each cycle of fatty acid elongation. Thereby, it participates in the production of VLCFAs of different chain lengths that are involved in multiple biological processes as precursors of membrane lipids and lipid mediators. . [Isoform 2]: In tooth development, may play a role in the recruitment and the differentiation of cells that contribute to cementum formation. May also bind hydroxyapatite and regulate its crystal nucleation to form cementum. .