Unlike MPP+, its common biological precursor MPTP is a lipid-soluble molecule that diffuses readily across the blood-brain barrier.[1] MPTP itself is not cytotoxic, however, and must be metabolized to MPP+ by MAO-B to show any signs of toxicity.[9] The oxidation of MPTP to MPP+ is a process that can be catalyzed only by MAO-B, and cells that express other forms of MAO do not show any MPP+ production.[9] Studies in which MAO-B was selectively inhibited showed that MPTP had no toxic effect, further cementing the crucial role of MAO-B in MPTP and MPP+ toxicity.[11]