DENVER (AP) — Mila Makovec has high pigtails in her dark hair and a cloth doll tucked under her arm as she wakes up in a hospital bed, where she's just been injected with a one-of-a-kind drug intended to save her life.

The drug works for only one person in the world — this 9-year-old girl from Boulder.

In a spectacular example of what the future might hold for precision medicine, the drug was made only for her in a quest to save Mila from a neurological disease that is destroying her brain. Her DNA is in the formula. The 22-letter genome sequence in the drug's recipe matches the one in Mila's...