Note that DG sym from the RNA/RNA calculation is subtracted because a 2 0 -O-methyl RNA/RNA duplex cannot be selfcomplementary because the backbones differ. For the duplexes studied here, the number of phosphate pairs is one less than the number of base pairs. The effects of LNA substitutions are likely not very dependent on salt concentration. Thus, it is probable that in 1 M NaCl or in the presence of Mg 2+ (70) that DG 37 (2 0 -O-MeRNA/ RNA) can be approximated by DG 37 (RNA/RNA, 1 M NaCl). Table 3 compares measured values for duplexes with more than one LNA to predictions from combining Equation 1-3. The measured DG 37 values average À10.5 kcal/mol and the root-mean-square difference between measured and predicted DG 37 values is 0.6 kcal/mol with the largest difference being 1.7 kcal/mol. Again, the sequence with the largest difference contains a 5 0 G M U L /3 0 CA nearest neighbor so the prediction would be improved if Equation 1 was corrected for the apparent instability of this motif.