Paternity testing bone marrow?

I have taken two paternity tests and both tests show that I am not the father of the child. The test shows that I match 5 of 15 of the sequences and gives me a probibility of 0.0%. The mother still denys that there is any possibility that the child could be someone elses. She claims that it is rare but that the tests are wrong and is suggesting a bone marrow test be done. Everything that I have read suggests that the possibilities are slim that the child is mine even if the genes have mutated. I want to know any information about bone marrow paternity testing. I would also like to know how it could be possible that a gene would mutate and why?