From a newsgroup post dated July 27, 2010:
Here’s another story idea that’s annoying me: Aliens come to Earth to exterminate a deadly threat to their civilization. They can’t explain how they know, but they’re absolutely certain that if they don’t wipe out every single carrier of a particular gene sequence, in about a thousand years their species will be extinct. They don’t want to hurt anyone else; in fact, they’ll be happy to give humanity their FTL drive and lots of other advanced tech and other goodies, and to welcome Earth into the galactic community — as soon as That Gene Sequence is gone. They’ll pay bounties for carriers, destroy entire cities that shield carriers, whatever it takes.
Some of the carriers are apes; in fact, a couple of endangered species are going to be completely wiped out.
And some of the carriers are human, which is where the story comes from.