Ultracentrifuged exosome samples were used for nanoparticle tracking analysis (NTA), as previously described (101). Briefly, isolated exosomes were resuspended in 500 to 1000 μl of PBS from which about 300 μl was loaded into the sample chamber of an LM10 unit (NanoSight, Amesbury, UK) using a disposable syringe. Sample durations of 30 to 60 s per sample were analyzed with NTA 2.3 software (NanoSight). Samples containing higher numbers of exosomes were diluted before the analysis, and their relative concentrations were then calculated according to the dilution factor. The size distribution of exosomes (fig. S10) was also determined using this method.