Methylmalonylcarnitine is an acylcarnitine. More specifically, it is an methylmalonic 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. Methylmalonylcarnitine is therefore classified as a short chain AC. As a short-chain acylcarnitine methylmalonylcarnitine is a member of the most abundant group of carnitines in the body, comprising more than 50% of all acylcarnitines quantified in tissues and biofluids (PMID: 31920980). Some short-chain carnitines have been studied as supplements or treatments for a number of diseases, including neurological disorders and inborn errors of metabolism. In particular methylmalonylcarnitine is elevated in the blood or plasma of individuals with methylmalonyl-coa epimerase deficiency/methylmalonic acidemia (PMID: 27699154, PMID: 17279485), familial mediterranean fever (PMID: 29900937), and cobalamin C deficiency (PMID: 25367534). It is also decreased in the blood or plasma of individuals with melanoma (PMID: 30830422) and intracerebral hemorrhage (PMID: 29265114). Methylmalonylcarnitine is elevated in the urine of individuals with methylmalonyl-coa epimerase deficiency/methylmalonic acidemia (PMID: 27699154, PMID: 17279485). Carnitine acetyltransferase (CrAT, EC:2.3.1.7) is responsible for the synthesis of all short-chain and short branched-chain acylcarnitines (PMID: 23485643). 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].