E-64 is an irreversible, cell-permeable, potent, and highly selective cysteine protease inhibitor. E-64 will not inhibit serine proteases (except trypsin) like other cysteine protease inhibitors, leupeptin and antipain. A very useful cysteine protease inhibitor for use in in vivo studies because it has a specific inhibition and displays low toxicity. MW: 357.41. Purity ≥99% by HPLC