Chlorotoxin I-131 is a synthetic version of a neurotoxin investigated in clinical trials for treating brain cancer. Chlorotoxin was originally isolated from the venom of the Israeli scorpion Leiurus quinquestriatus, and has potential as a tumor imaging agent based on its selective binding to tumor cells. Molecular targets for Chlorotoxin include voltage-gated chloride channels (GCC), calcium-dependent phospholipid-binding protein Annexin-2, and the inducible extracellular enzyme Matrix Metalloproteinase-2 (MMP-2). Chlorotoxin reduced the migration ability of glioma cells through tight extracellular spaces in the brain tissue by inhibition of the MMP2 because this prevented the cells from shrinking and releasing from the extracellular matrix. Chlorotoxin is a small 36-amino-acid peptide that selectively binds to glioma cells but not normal brain parenchyma. The synthetic version of this peptide has been manufactured and covalently linked to iodine 131 as a means of targeting radiation to tumor cells in the treatment of brain cancer.