  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.
