Fatal familial insomnia steals the ability to fall asleep, reports NPR.

Basically, certain molecules had started clumping together in Kamni’s brain, killing her brain cells. It was all because of one tiny error in her DNA — an “A” where there was supposed to be a “G,” a single typo in a manuscript of 6 billion letters.

Sonia sent a sample of her own blood to a lab, where a test confirmed she inherited the same mutation. The finding threw the family into grief all over again.