To evolve the virus best suited for gene therapy, Berkeley researchers created more than 100 million engineered adeno-associated viruses (left) and injected them into the gel-like center of the eye. Those able to penetrate the many cell layers of the retina are used to shuttle a corrective gene into cells with a defective gene. To reach these cells, in particular the light-sensitive photoreceptors and the retinal pigment epithelium (RPE) cells, the virus must pass through multiple cell layers (right).