What are their biological and clinical significance? Some functional roles ascribed to exosomes include cellular communication, immune system modulation and tumor progression. (Stoorvogel et al., 2002 The biogenesis and functions of exosomes; Al-Nedawi et al., 2009 Microvesicles: messengers and mediators of tumor progression.) Research has indicated that circulating exosome levels have also been found to be elevated in diseased states such as ovarian cancer (Taylor et al., 2008 MicroRNA signatures of tumor-derived exosomes as diagnostic biomarkers of ovarian cancer), lung cancer (Rabinowtiz et al., 2009 Exosomal microRNA: a diagnostic marker for lung cancer) and melanoma (Logozzi et al., 2009 High levels of exosomes expressing CD63 and caveolin-1 in plasma of melanoma patients).  This suggests that exosomal load assessment and exosomal molecular profiling hold great promise for disease detection and monitoring.  Exosomes also appear to play a significant role in cellular communication as they transfer RNAs including mRNA and microRNA as well as protein to neighboring cells (Valadi et al., 2008 Exosome-mediated transfer of mRNA and microRNA is a novel mechanism of genetic exchange; Al-Nedawi et al., 2008 Intercellular transfer of the oncogenic receptor EGFRvIII by microvesicles derived from tumour cells).