These are the brain cells in a mouse's hippocampus which were targeted either by a naturally occurring AAV9 capsid (red) versus an engineered capsid (gray). Typically, the immune system protects against sequential viral infections, but by selecting sufficiently distant capsid lineages, we were able to circumvent this limitation. This is a critical result for our AAV screening strategy, as we intend to first dose a "humanizing" AAV2-based capsid, and subsequently screen an array of AAV9-based capsids in the same mouse.