Popular Science says:

All the new breakthroughs in microscopy we’ve seen recently are designed to help scientists see deeper, inside individual cells and into the depths of the brain. Of course, this would be easier to do if there wasn’t a bunch of other tissue blocking the cells you want to see. Japanese researchers have a new solution: Make it all transparent.

A new chemical reagent makes the brain see-through, allowing fluorescent tags to light up neurons and blood vessels deep inside. This enables 3-D images of entire structures, without having to cut anything away or divide anything into smaller sections.

Start your invisible assassin screenplays now, kids.