Achieving cell-type specificity for opsin production is necessary because different cell types are jumbled together in the same brain regions.

“because well even if you're just getting the cells the cell bodies that are there there are many different kinds there are the dopamine cells but right next to them there are the gaba cells and next to them are the glutamate cells and and they're all jumbled up together and there you're not too different from the electrode now if you put in light you're still going to be stimulating all these cells and so what you need is a way to make the production of the opsin cell type specific”