Brain cells are protected from possible contamination by substances in circulating blood by what is known as “the blood-brain barrier.” Researchers have many questions about precisely how this protective mechanism works. Recently, Harvard Medical Sschool researchers identified a protein that supports the blood-brain barrier. When a molecule, apolipoprotein E (apoE), is absent, the barrier becomes especially porous, making the brain vulnerable to trauma and possibly Alzheimer’s disease. Denisa Wagner, Harvard Medical School professor of pathology at the Center for Blood Research and senior author of the study, “wondered whether apoE might be important for the integrity of the brain’s vasculature.” In a study with mice, her research team found that it was. According to Wagner, this study, published in the December 2001 issue of Molecular Medicine, and two others published recently suggest links between apoE, an impaired blood-brain barrier, and Alzheimer’s. Major risk factors for the disease, such as brain injury, age, and high cholesterol, are precisely those that aggravate the increase in blood-brain barrier permeability associated with apoE deficiency.