  We consider DNA codes based on the nearest-neighbor (stem) similarity model
which adequately reflects the "hybridization potential" of two DNA sequences.
Our aim is to present a survey of bounds on the rate of DNA codes with respect
to a thermodynamically motivated similarity measure called an additive stem
similarity. These results yield a method to analyze and compare known samples
of the nearest neighbor "thermodynamic weights" associated to stacked pairs
that occurred in DNA secondary structures.
