We have mechanistically implicated a newly defined pro-angiogenic miRNA, let-7b-5p, in the proangiogenic responses to PF exosomes. However, in addition to let-7b-5p and other proangiogenic miRNAs, PF exosomes also contain several anti-angiogenic miRNAs (Table S3). The overall proangiogenic responses to the PF exosomes might be dictated by a functional prevalence of proangiogenic miRNAs that are simultaneously transferred by the same nanoparticles. However we cannot discount the possibility that anti-angiogenic miRNAs are enriched in a subset of exosomes that are taken up with reduced efficiency by the ECs. Moreover, the expression of individual miRNAs in a cell context and possibly in the exosomes does not always reflect the level of the individual miRNAs binding to AGO-2 and hence their functionality. In fact, it is emerging that the association between miRNAs and the RISC complex is a regulated process. As an example, Krell et al.33 showed that following DNA damage, P53 interacts with AGO-2 to induce or reduce AGO-2’s association with subsets of miRNAs, including multiple let-7 family members.