Figure A.

To form a barrier against xenobiotics, the BBB employs two main modes of exclusion: a) tight paracellular junctions that exclude charged molecules (blue) and b) a wide array of drug transporters that efflux charge neutral, lipophilic compounds back into the vacular space (red). A charge neutral dye is not excluded from W.T. or mutant animals as it can take a transcellular route to entry across the BBB.