Channelrhodopsins require high numbers to be packed into cells without causing damage due to their small current and low conductance.

“These channelrhodopsins actually don't move many ions. They have a small current, small conductance, as we say. And so we had to figure out ways to pack a lot of them into cells without damaging cells, and still make them targetable, so we don't want them to just be in all the cells, 'cause then it becomes just like an electrode. You're just stimulating all the cells that are nearby. We had to keep that specificity, make them targetable to just one kind of cell or another, while still packing in large numbers of them into those cells.”