"It's matching the species code, not the entire fingerprint. The boy didn't turn into his mother, just back into a human. And as far as they're concerned, you're halfway there. Your DNA has temporal damage from the vortex. It might be enough."
The Doctor walked back to her, gently putting his hands on her shoulders. "The nanogenes will stay here. It might not even work, but we'd be in no worse shape if it doesn't. And you wouldn't change outwardly at all, I should think. Nothing wrong there, just at the cellular level. You wouldn't turn into my big eared sister."
He grinned, but his eyes were serious. "It's your choice, Rose."