Apo B exists in human plasma as two isoforms: apo B48 and apo B100. Apo B100 is the major physiological ligand for the LDL receptor. It contains 4536 amino acid residues. Apo B100 is the full length protein. Its gene has been mapped on the short arm of chromosome 2, with an approximate length of 43 kilobases and 29 exons. The LDL-binding domain of the molecule is proposed to be located between the aa3129-3532. Apo B100 is synthesized in the liver. It is required for the assembly of very low density lipoproteins (VLDL). It does not interchange between lipoprotein particles, as do the other lipoproteins. It is found in IDL and LDL particles after the removal of the apolipoproteins A, E and C. Apo B48 is present in chylomicrons and chylomicron remnants. It plays an essential role in the intestinal absorption of dietary fats. Apo B48 is synthesized in the small intestine. It comprises the N-terminal 48% of apo B100. Apo B48 is the degraded protein. Apo B48 lacks the LDL receptor-binding domain of Apo B100. It is produced due to post-transscriptional Apo B100 mRNA editing at codon 2153, which creates a stop codon in the intestine instead of a glutamine in the liver.