Genetic Variability of Splicing Sites
  Splicing sites provide unique statistics in human genome due to their large number and reasonably complete annotation. Analyses of the cumulative SNPs distribution in splicing sites reveal a few interesting observations. While a degree of the nucleotide conservation reflects on the SNPs density monotonically, no detectable changes in the SNPs frequencies spectrum were found. Semi-conserved nucleotide sites harbor transition mutations predominantly. We propose that such transition preference is caused by co-evolution of a site with corresponding binding agents. Since transitions in humans and similarly in other organisms are almost twice as frequent as transversions, this adaptation significantly lowers the mutation load. 
Dmitri Parkhomchuk