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Bio-objects as “boundary crawlers: ” the case of microRNAs Abstract microRNAs (miRNAs), short RNAs of 21-25 nu-cleotides, are implied in gene expression and regulation, in biological processes and in human pathologies includ-ing cancer. Since miRNAs of plant origin can survive diges-tion and cooking and enter in animal (including human) sera and tissues, their intervention in mammalian gene expression and regulation might be expected. Mouse ex-perimental feeding, in fact, showed that a miRNA class (MI-R168a) is involved in accumulation of the low-density li-poprotein (LDL), the major cholesterol-carrying lipoprotein of human plasma. Considering LDL’s role in atherosclerosis, a negative influence of miRNAs from food origin on our health may be expected. Here we concentrate on the miR-NAs ’ capability to cross inter-kingdom boundaries through the diet and acting as a “boundary crawler. ” The boundary between plant and human is presented under a new per-spective, where a new intimate relationship between two genomes – mammalian and plant – belonging to quite dif-ferent kingdoms is proposed. The food’s role as molecule carrier in our health is also discussed. miRNAs, finally, are presented as an example of “bio-objects ” with impact on both medical and cultural issues. In this essay in a series of articles from “Bio-Objects ” re-search network supported by the Cooperation in Science and Technology (COST) program (1), we present an ex-ample of a bio-object: microRNAs (miRNAs). They belon