Internal mismatches make duplex formation less favorable Table 5 contains thermodynamic parameters measured for the formation of duplexes containing single mismatches and the difference in stabilities relative to completely Watson-Crick complementary duplexes (Tables 1 and 2 ). All internal mismatches make duplex formation less favorable by at least 2 kcal/mol at 37 C corresponding to at least a 25-fold less favorable equilibrium constant for duplex formation. In general, terminal mismatches destabilize much less than internal mismatches. In fact, when the 3 0 terminal U L of 5 0 A M C M U M A M C M C M U L makes a GU pair, the duplex is stabilized by 0.14 kcal/mol at 37 C relative to a terminal AU pair.