APOE, or apolipoprotein E, is a protein involved in lipid metabolism and is crucial for transporting fats in the bloodstream. It plays a significant role in the brain, where it helps maintain the health of neurons and supports the repair of damaged tissues. There are three common variants of APOE: ε2, ε3, and ε4. The ε4 variant is associated with an increased risk of developing Alzheimer's disease, while the ε2 variant may offer some protective effects. Understanding APOE is important for research into neurodegenerative diseases and potential treatments.