Viruses are nature’s Trojan horses: They gain entrance to cells, smuggle in their genetic material, and use the cell’s own machinery to replicate. For decades, scientists have studied how to repurpose these invaders to deliver therapeutics for treating disease and tools for studying cells. Researchers in the lab of Viviana Gradinaru, professor of neuroscience and biological engineering, Heritage Medical Research Institute Investigator, and director of the Center for Molecular and Cellular Neuroscience at the Tianqiao and Chrissy Chen Institute for Neuroscience at Caltech, have now developed a method to rapidly and efficiently identify designer adeno-associated virus (AAV) variants that can deliver to or “transduce” specific types of cells in mice, enabling scientists to choose a virus based on research or clinical needs.