À0.73 ± 0.26 5 0 AU3 0 À1.10 ± 0.08 Similar sequence dependent parameters may also be applicable to 2 0 -O-methyl RNA/RNA heteroduplexes because they are expected to have A-form conformations similar to those of RNA/RNA homoduplexes (68) . This was tested by comparing the predicted stabilities of RNA/RNA duplexes in 1 M NaCl at 37 C with those measured for 2 0 -O-methyl RNA/RNA duplexes in 0.1 M NaCl at 37 C. The predicted thermodynamics are listed in parentheses in Tables 1 and 2 . On average at 37 C, the RNA/RNA duplexes in 1 M NaCl are 0.12 ± 0.01 kcal/mol of phosphate pairs more stable than the 2 0 -Omethyl RNA/RNA duplexes in 0.1 M NaCl. Presumably, much of this difference is due to a sequence independent effect of salt concentration, which would primarily affect the DS for duplex formation (22, 69) . Thus, a reasonable approximation for the first term on the right hand side of Equation 1 is: