(A) A given region of mRNA (pink) should undergo random base-pairing interactions with a certain number of noncoding RNAs (orange). However, a phenomenon called "avoidance" results in the number of observed pairings (right) being fewer than the number expected by chance (left). This simplified picture does not allow for the influence of binding energies and other effects. (B) The ability of RNA molecules to fold into complex structures means that mRNA-noncoding RNA interactions can have a range of binding energies (see color bar). "Avoidance" results in the observed binding energies (right) being, on average, weaker than the binding energies expected by chance (left). Umu et al. report a significant difference in binding energy distributions in 73% of bacterial and archeal species. (C) If a random base-pairing interaction results in a noncoding RNA pairing with part or all of a start codon in an mRNA, the ribosome will not be able to translate the mRNA.