6-hydroxyoct-2-enedioylcarnitine is an acylcarnitine. More specifically, it is an 6-hydroxyoct-2-enedioic acid ester of carnitine. Acylcarnitines were first discovered more than 70 year ago (PMID: 13825279). It is believed that there are more than 1000 types of acylcarnitines in the human body. The general role of acylcarnitines is to transport acyl-groups (organic acids and fatty acids) from the cytoplasm into the mitochondria so that they can be broken down to produce energy.  This process is known as beta-oxidation. According to a recent review [Dambrova et al. 2021, Physiological Reviews], acylcarnitines (ACs) can be classified into 9 different categories depending on the type and size of their acyl-group: 1) short-chain ACs; 2) medium-chain ACs; 3) long-chain ACs; 4) very long-chain ACs; 5) hydroxy ACs; 6) branched chain ACs; 7) unsaturated ACs; 8) dicarboxylic ACs and 9) miscellaneous ACs. Short-chain ACs have acyl-groups with two to five carbons (C2-C5), medium-chain ACs have acyl-groups with six to thirteen carbons (C6-C13), long-chain ACs have acyl-groups with fourteen to twenty once carbons (C14-C21) and very long-chain ACs have acyl groups with more than 22 carbons. 6-hydroxyoct-2-enedioylcarnitine is therefore classified as a medium chain AC. As a medium-chain acylcarnitine 6-hydroxyoct-2-enedioylcarnitine is somewhat less abundant than short-chain acylcarnitines. These are formed either through esterification with L-carnitine or through the peroxisomal metabolism of longer chain acylcarnitines (PMID: 30540494). Many medium-chain acylcarnitines can serve as useful markers for inherited disorders of fatty acid metabolism. Carnitine octanoyltransferase (CrOT, EC:2.3.1.137) is responsible for the synthesis of all medium-chain (MCAC, C5-C12) and medium-length branched-chain acylcarnitines in peroxisomes (PMID: 10486279). The study of acylcarnitines is an active area of research and it is likely that many novel acylcarnitines will be discovered in the coming years. It is also likely that many novel roles in health and disease will be uncovered. An excellent review of the current state of knowledge for acylcarnitines is available at [Dambrova et al. 2021, Physiological Reviews].