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Lipids such as cholesterol are insoluble in plasma, and for delivery to tissues such as the adrenal gland, gonads, etc., they have to be packaged into lipoproteins with cholesterol esters and triglycerides in the core and phospholipids, free cholesterol, and apolipoproteins on the surface. Such is the composition of the lipoproteins.[1]  Apolipoprotein B (ApoB) is the primary apolipoprotein and is the carrier for the following lipids: chylomicrons, low-density lipoprotein ( LDL), very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and lipoprotein (a).  ApoB is not found in high-density lipoproteins (HDL). The latter are reconstituted into lipoproteins with Apo A.  Hepatic ApoB has a molecular mass of 540000 Da.  There are two circulating forms of Apo B, Apo B48 (from the small intestine) and Apo B100 (from the liver).[1] Intestinal ApoB, which is present in chylomicrons, has a molecular mass of 48% of that of hepatic ApoB.  Hence, hepatic ApoB is termed ApoB100, and intestinal Apo B as apoB 48. The same gene codes for both ApoB48 and ApoB 100.  Apo B-100 contains 4536 amino acids and is necessary for the assembly of VLDL in the liver and also serves as the primary ligand for LDL receptor-mediated clearance of LDL particles from the blood. Apo B-48 has 2512 amino acids and is essential for the formation of chylomicrons, and serves in the absorption of dietary fats from the intestine.

Familial defective apo B is an autosomal dominant disorder that arises due to a mutation in apo B that prevents binding of the defective apo B to the LDL receptor. This condition results in a clinical phenotype similar to classical familial hypercholesterolemia due to mutations in the LDL receptor with increased LDL-cholesterol, xanthomas, and premature ASCVD, underscoring the importance of apoB in atherogenesis. The most frequent variant is apoB-3500, caused by a point mutation resulting in a substitution of glutamine for arginine at position 3500.[1] This mutation is an excellent illustration of a defect in an important ligand resulting in premature ASCVD.