Just a quick note for the reference.
Methylated CpG sequences in DNA (C-G sequences noticed as CpG to make them apart from C-G bonds between the two 'mirror' DNA chains) have a much faster mutation rate than all other DNA. This is because the group, apparently related to the immune system often, is most unstable what yields a mutation rate 10-50 times faster than for the rest of DNA.
This may have implications for the molecular clock hypotheses but I'm not sure in which sense right now.
More information at Science Daily.
Ref. J.C. Walser and A. Furano, The mutational spectrum of non-CpG DNA varies with CpG content. Genome Research 2010. Pay per view.