When I brought last TB she had a on going DNA query. The previous owner had bred from her and wanted to find her blood lines.
They were able to match her blood to that of a foal they had registered many uears previously.
We had to on our TB to prove she was who we said she was for her passport. Previous owners didn;t give us her papers so we had to reapply to get a passport - this was just as passports were being introduced.
I knew the dam and sire though, I worked on the yard that bred her, we'd bought her from there, sold her on, then got her back!
Not exactly cheap to do the tests, and luckily they came back exactly as we'd said.