FIGURE 10-28 Base pairing with target RNAs distinguishes miRNA and siRNA. (a) miRNAs hybridize imperfectly with their target mRNAs, repressing translation of the mRNA. Nucleotides 2–7 of an miRNA (highlighted blue) are the most critical for targeting it to a specific mRNA. The CXCR4 miRNA shown at the bottom is a synthetic oligonucleotide introduced into cells by transfection. (b) siRNA hybridizes perfectly with its target mRNA, causing cleavage of the mRNA at the position indicated by the red arrow, triggering its rapid degradation. See P. D. Zamore and B. Haley, 2005, Science 309:1519.