Scientists uncovered a surprising four-layer structure hidden inside the hippocampal CA1 region, one of the brain’s major centers for memory, navigation, and emotion. Using advanced RNA imaging techniques, the team mapped more than 330,000 genetic signals from tens of thousands of neurons, revealing crisp, shifting bands of cell types that run along the length of the hippocampus. This layered organization may help explain why different parts of CA1 support different behaviors and why certain neurons break down more easily in disorders such as Alzheimer’s disease and epilepsy.