Primary carnitine deficiency (PCD, OMIM212140) is a fatty acid oxidative metabolic disease caused by defective function of carnitine transporter OCTN2, which is an autosomal recessive disorder. PCD was first reported in 1975, and 13 years later, a study suggested that PCD is caused by a defective carnitine transporter OCTN2. The study proposed that PCD was due to a cell membrane carnitine transport disorder, and the gene for PCD was localized in 1998, and since then, PCD has been increasingly reported. With the application of tandem mass spectrometry, the diagnosis rate of newborn screening and clinical high-risk has significantly improved; the intervention of L-carnitine replacement therapy has improved the prognosis of an increasing number of PCD patients. Meanwhile, the research on the pathogenesis of PCD and gene mutation has been intensifying. Carnitine (i.e., 3-hydroxy-4-trimethylaminobutyric acid) in nature has two forms, levocarnitine and dextro-carnitine, of which levocarnitine is physiologically active. Approximately 75% of carnitine in the body comes from food and 25% is synthesized by the liver and kidneys. Under normal conditions, L-carnitine enters cells (except hepatocytes) through the action of OCTN2, a high-affinity transporter on cell membranes, and is most abundant in cardiac and skeletal muscle. The b-oxidative metabolism of fatty acids in vivo is carried out in mitochondria. Intracellular long-chain fatty acids cannot enter mitochondria directly, and their activated form, long-chain lipoyl CoA, is catalyzed by carnitine palmitoyltransferase I in the outer mitochondrial membrane to combine with carnitine to produce lipoyl carnitine, which enters the mitochondrial matrix under the action of carnitine lipoyl carnitine translocase in the inner mitochondrial membrane, and subsequently under the action of carnitine palmitoyltransferase II in the inner mitochondrial membrane The carnitine is then decomposed into long-chain lipid acyl-CoA and free carnitine by the action of carnitine lipid acyl-carnitine translocase in the inner mitochondrial membrane, and the released carnitine is transferred out by the action of carnitine lipid acyl-carnitine translocase and recycled, and the medium and short chain fatty acids can enter the mitochondria directly. In addition, carnitine can reduce the acetyl CoA/CoA ratio, increase the activity of pyruvate dehydrogenase complex and promote the oxidative utilization of glucose; it can also increase the activity of mitochondrial respiratory chain enzyme complex and accelerate the production of ATP. In addition, carnitine also has physiological effects such as regulating immune response, antioxidant, inhibiting apoptosis, maintaining mitochondrial ultrastructure and membrane stability.OCTN2 is a Na+-dependent organic cation/carnitine transporter, consisting of 557 amino acids and containing 12 transmembrane regions, which is present in cell membranes of cardiac muscle, skeletal muscle, small intestine, renal tubules, skin fibroblasts and placenta and other tissues. Some scholars have studied its functional domains and found that: the transmembrane region plays a key role in carnitine recognition and transport; the N-terminus (amino acid sequence 1-193) may have a Na+ binding site; the C-terminus (amino acid sequence 342-557) is related to the transport of Na+-carnitine complex, where the intracellular loop located between transmembrane region 10 and transmembrane region 11 is coupled to the Na+ electrochemical gradient and the Na+-carnitine complex across the cell membrane, and tyrosine residues in this loop play an important role. Mutations in its related genes result in defective OCTN2 carnitine transport, which prevents carnitine from entering into the cell and reduces carnitine absorption through the intestine, with a corresponding reduction in free carnitine in body fluids. At the same time, the renal tubular carnitine reabsorption disorder leads to increased urinary carnitine excretion and decreased plasma carnitine level, and the intracellular carnitine is even more deficient, resulting in long-chain fatty acids not entering the mitochondria for β-oxidation and reduced acetyl CoA production, resulting in insufficient energy supply when the body needs fat mobilization for energy supply, and a large accumulation of lipids in the cells; in addition, carnitine deficiency also indirectly affects glucose and other metabolic pathways. Ultimately, it causes damage to the body, especially when fasting or stress.