It’s well known that carrying one copy of the APOE4 gene variant increases one’s risk for Alzheimer’s disease threefold and two copies about tenfold, but the fundamental reasons why and what can be done to help patients remain largely unknown. A study published by an MIT-based team Nov. 16 in Nature provides some new answers as part of a broader line of research that has demonstrated APOE4’s consequences cell type by cell type in the brain.