Apolipoproteins are lipid transport proteins, helping to move lipids (and fat-soluble vitamins) in the body. They are produced in the small intestine and the liver and each apolipoprotein have a specific function in lipid transport and metabolism. Apolipoprotein A (Apo A) is the primary portion of the high-density lipoprotein (HDL) which plays an important role in transporting cholesterol from the peripheral tissues to the liver, where it can be excreted. Apo B, specifically the form ApoB-100, is the main lipoprotein of low-density lipoprotein (LDL) and which are the primary driver of plaques that cause vascular disease. Apo E is a major component of HDL-like particles in the brain and high levels are associated with increased all-cause, cardiovascular, and cancer mortality. The ε4 allele of Apo E is also the major genetic risk factor for Alzheimer's disease. Diagnostic assays that measure the levels of Apo B-100 help determine the risk of developing cardiovascular disease and sometimes help monitor treatment for high cholesterol. Apo A testing is typically done in combination with Apo B as apo B/apo A1 ratio has been shown to be predictive of ischemic stroke in patients. Apo E levels are measured for diagnosing hyperlipoproteinemia in patients with increase plasma triglycerides and cholesterol.