L17E, a cationic amphiphilic peptide with endosomolytic properties, selectively disrupts the membranes of late endosomes (LEs) in an acidic environment and prevents TS5-p45-induced apoptosis in a dose-dependent manner. This peptide is internalized by cells and directed to LEs, where it compromises LE integrity and facilitates the release of their contents into the cytosol. L17E serves as a tool for probing the endosomal protein trafficking pathway [1].