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In plasma and other body fluids, miRNAs can be found. They not only originate from dying cells but also from active secretion and are usually 'packed' into vesicles/lipo-proteic structures (i.e. exosomes) protecting them from the action of ribonucleases. Thus, miRNAs in plasma are very stable, offering diagnostic opportunities.

My question is whether they may also have a functional role. Exosome transfer between cells has been demonstrated, so it is likely that miRNAs can be secreted by one source cell and reach a target cell, similarly to how hormones works. However, this has not been quantitatively assessed. Is the concentration of miRNAs in plasma known? Is it sufficient to hypothesize functional regulation of gene expression in target cells?

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See papers by D Bianchi for reports of miR levels in blood of pregnant humans, both of maternal and fetal origin.

A review by Steer and Subramanian offers a number of examples pertaining to your question.

Lastly, Vickers, Palmisano, et al. (2011) show that some miRs are present in the HDL-cholesterol particle and that those levels differ between normal and familial hypercholesterolemia human subjects.

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