The parents who created a drug to save their daughters
15 March 2017 Last updated at 00:11 GMT
When Chris and Hugh Hempel were told their twin daughters had a fatal disease that couldn't be treated, they decided there was only one thing they could do - create a new medicine themselves.
Now they're giving hope to other families too.
Chris and Hugh's story is featured in #IncredibleMedicine at 21:00 on BBC Two on Wednesday 15 March. Catch up later online.