MicroRNAs (miRNAs) are short RNA sequences that repress protein synthesis by either inhibiting the translation of messenger RNA (mRNA) or increasing mRNA degradation. Endogenous miRNAs have been found in various organisms, including animals, plants, and viruses. Mammalian miRNAs are evolutionarily conserved, are scattered throughout chromosomes, and play an important role in the immune response and the onset of cancer. For this study, the author explored the base composition characteristics of miRNA genes from the six mammalian species that contain the largest number of known miRNAs. It was found that mammalian miRNAs are evolutionarily conserved and GU-rich. Interestingly, in the miRNA sequences investigated, A residues are clearly the most frequent occupants of positions 2 and 3 of the 5′ end of miRNAs. Unlike G and U residues that may pair with C/U and A/G, respectively, A residues can only pair with U residues of target mRNAs, which may augment the recognition specificity of the 5′ seed region. 1. Introduction MicroRNAs (miRNAs) are single-stranded, short (15–27 nucleotides) RNA sequences that repress protein synthesis via base pairing to some portion of a messenger RNA (mRNA), such as the 3′ untranslated region (3′UTR), the 5′ untranslated region (5′UTR), or the coding region [1, 2]. Endogenous miRNAs have been found in various organisms, including animals, plants, and viruses. In mammals, precursor miRNAs (pre-miRNAs) 60–100 nucleotides in length form a hairpin stem-loop structure and are processed by Dicer, a highly conserved RNase III family endonuclease that is found in almost all eukaryotic organisms [3, 4]. This processing yields a miRNA？: miRNA duplex that is approximately 21 nucleotides long. One or both of the duplex’s short RNA strands is incorporated into RNA-induced silencing complexes (RISCs, which are composed of Dicer, Argonaute, and other nonspecified proteins) and functions as a mature miRNA that can base pair with mRNA targets, inducing either the degradation or translational repression of the mRNA [3, 4]. The regulation of miRNA is essential for organisms because miRNA plays critical roles in numerous biological processes, including the proliferation and differentiation of cells, as well as apoptosis. It is predicted that mammalian miRNAs can regulate approximately 30% of protein-coding genes [4]. The dysfunctional posttranscriptional modulation of gene expression caused by miRNAs has been related to diseases such as cancer and neurodegenerative disorders [5–8]. The investigation of miRNAs and their gene targets has thus
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