Depending on the complementarity between miRNA and 3' untranslated region (UTR) of target mRNA there are two known mechanisms of miRNAs action on mRNAs: 1) target mRNA degradation and 2) translational inhibition with little or no influence on mRNA levels [24] (Figure 2) . Firstly, the deadenylation and subsequent degradation of the target mRNA occurs when miRNA is near-perfectly complementary with target mRNA [25, 26] . A recent study proved that mRNA degradation represents the major mechanism of miRNA regulation [27] . The authors showed that about 84% of all protein-coding mRNA targets undergo degradation while recognized by their cognate miRNA [27] . Secondly, the translational inhibition MiRNAs are transcribed by RNA polymerase II from the genomic DNA as long (hundred or thousand nucleotides) primary miRNA transcripts (pri-miRNAs). A local stem-loop structure of pri-miRNAs is then cleaved in the nucleus by the dsRNA-specific ribonuclease Drosha/Pasha to produce a 70 nucleotides long precursor miRNA (pre-miRNA). Pre-miRNAs in form of hairpins are then actively transported from the nucleus to the cytoplasm. In the cytoplasm, pre-miRNAs are subsequently cleaved by RNase III Dicer into~22-nt miRNA duplexes, consisting of the "guide" (miR) strand and the "passenger" (miR*) strand. The "passenger" strand is degraded, the "guide" strand is incorporated into the RNA-induced silencing complex (RISC) and serves as a functional, mature miRNA, acting by two different mechanisms according to the complementarity with the target mRNA. Adopted from Kim [15] . occurs when miRNA is only partially complementary to its target mRNA [28] [29] [30] . In light of the recent study by Guo et al [27] , this mechanism does not represent a predominant reason for reduced protein output.