Hi, is there a specific exosome marker? this is a very important however difficult question to answer. To my knowledge, there have been many attempts to suggest a common exosomal marker(s). Yoshioka et al (J of Extracellular vesicles, 2013) suggested that CD9 and CD81 could be useful markers for the presence of EVs as they were conserved in several different cell lines. However, our own findings (oksvold et al 2014, Clinical Therapeutics - http://bit.ly/1zYrmcO) suggested that CD9 may not be such a general markers as we were not able to detect CD9 in pre-enriched exosomes from B-cell lymphoma cell lines which is compatible with the findings by Yoon et al (The American J. of Path 2010) showing loss of CD9 in B-cell lymphomas. The best recommendation I can give is to follow the recommendations suggested by Lotvall et al, 2014, J of Extracellular vesicles) who presents minimal experimental requirements for definition of EVs.