[5][6] Apolipoprotein B is the primary apolipoprotein of chylomicrons, VLDL, Lp(a), IDL, and LDL particles (LDL—commonly known as "bad cholesterol" when in reference to both heart disease and vascular disease in general), which is responsible for carrying fat molecules (lipids), including cholesterol, around the body to all cells within all tissues.

Through mechanisms only partially understood, high levels of ApoB, especially associated with the higher LDL particle concentrations, are the primary driver of plaques that cause vascular disease (atherosclerosis), commonly first becoming obviously symptomatic as heart disease, stroke and many other body wide complications after decades of progression.

However, primarily for historic cost/complexity reasons, cholesterol, and estimated LDL-cholesterol by calculation, remains the most commonly promoted lipid test for the risk factor of atherosclerosis.

Refined and automated NMR methods allow measurement distinctions between the many different ApoB particles.

[10] Abetalipoproteinaemia is caused by a mutation in the microsomal triglyceride transfer protein gene, MTTP.

[13] Mice containing only one functional copy of the mApoB gene show the opposite effect, being resistant to hypercholesterolemia.

There appears to be a trans-acting tissue-specific splicing gene that determines which isoform is ultimately produced.

[citation needed] Alternatively, there is some evidence that a cis-acting element several thousand bp upstream determines which isoform is produced.

[citation needed] As a result of the RNA editing, ApoB48 and ApoB100 share a common N-terminal sequence, but ApoB48 lacks ApoB100's C-terminal LDL receptor binding region.

The mechanism of antagonism entails binding ApoB, to a S. aureus autoinducer pheromone, preventing signaling through its receptor.

[16] Overproduction of apolipoprotein B can result in lipid-induced endoplasmic reticulum stress and insulin resistance in the liver.

In this way, one can understand that the number of ApoB100-containing lipoprotein particles which can carry lipids into the artery walls is a key determinant, driver of atherosclerosis and heart disease.

Since such competition will prolong the residence time of LDL particles in the circulation, it may lead to greater opportunity for them to undergo oxidation and/or other chemical modifications.

This occurs as a result of modified-LDL's toxic effect on vascular endothelium as well as its ability both to recruit immune effector cells and to promote platelet activation.

The INTERHEART study found that the ApoB100 / ApoA1 ratio is more effective at predicting heart attack risk, in patients who had had an acute myocardial infarction, than either the ApoB100 or ApoA1 measure alone.

[23]) In the general population this remains unclear although in a recent study ApoB was the strongest risk marker for cardiovascular events.

[31] The mRNA encoding this protein is subject to cytidine to uridine (C to U) site-specific RNA editing.

It acts as the RNA binding subunit and directs ApoBEC-1 to the ApoB mRNA downstream of the edited cytidine.

[46] A region called the spacer element is found 2–8 nucleotides between the edited nucleoside and this mooring sequence.

[49] The predicted secondary structure formed by ApoB mRNA is thought to allow for contact between the residue to be edited and the active site of APOBEC1 as well as for binding of ACF and other auxiliary factors associated with the editosome.

[51][52] Editing levels of ApoB mRNA have been shown to vary in response to changes in diet.

Recently the first structure of LDL at human body temperature in native condition has been found using cryo-electron microscopy at a resolution of 16 Angstrom.

[59] The function of this isoform is in fat absorption of the small intestine and is involved in the synthesis, assembly and secretion of chylomicrons.