I have been purifying exosomes using the Total Exosome Isolation (from cell culture media) kit and am able to identify several exosomal markers by western blot (CD63, Alix, TSG1010, HSP90, Hsc70). However I recently also found Calnexin in my preparation (by western blot). Have you seen this before using this kit? What does it signify? Are there other markers that are better to use as "negative" controls for western blot?