For this assay, exosomes obtained according to our method (see above) were treated as previously described.43 Isolated exosomes were incubated with proteinase K (50 μg/ml; Sigma) for 10 min at 37°C, before 5 mM phenulmethulsulfonyl fluoride (PMSF; Sigma) was added for 10 min at room temperature to inhibit the proteinase K (PK) activity. After the PK inactivation, the samples were incubated with 100 μg/ml RNase A (Thermo Fisher Scientific) for 20 min at 37°C to degrade unprotected RNA, followed by incubation with RiboLock RNase Inhibitor (Thermo Fisher Scientific). Finally protein or RNA was extracted as described earlier. As control samples we included (1) sonicated exosomes followed by treatment with PK and RNase A or (2) exosomes treated with PBS added instead of PK and RNase A.