Gene delivery is complicated because it's difficult for us to know how to change the properties of the virus. So, in this study, we did it the same way nature does it. By using random mutation, we were able to get about 10 million different variants. Now, we have a lot of versions of the virus to choose from. And we've selected about a half dozen of them that we are focusing on.

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This virus tends to stick to tissue rather than move through it, so when we inject it into an organ like the brain, for instance, it tends to stay there. To be successful, we need the virus to move and spread through the body. So, we have evolved variants [of the virus] that have less stickiness to tissue.

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