Mutations induced by ancient DNA extracts?

We have investigated whether some factor in ancient DNA extracts induces site-specific mutations in modern DNA. We find no evidence for higher mutation rates when extracts from three different Pleistocene mammals are added to modern DNA than when water or extraction blanks are added. We also fail to find evidence that any such factor affects ancient DNA sequences determined from the same extracts. This as well as the patterns of nucleotide substitutions seen in DNA sequences determined from hundreds of other specimens leads us to doubt that a previously unknown mutagenic factor can be a general feature of extracts from old tissues.