If this particular drug, or any other one, gets to clinical trials, it will face another set of hurdles. A typical trial for a drug treating a common disease like diabetes might involve thousands of patients. That scale makes it possible to run statistical tests ensuring that the drug really is effective. It also allows scientists to detect side effects that might affect relatively few patients. But even if you enrolled every FOP patient in the United States, a trial would still be a fraction of the size of a conventional one.

In recent years, the FDA has responded to this bind by smoothing out the approval of drugs for rare diseases. If doctors can’t find thousands of patients to enroll in a clinical trial, they are now allowed to conduct smaller trials that meet certain guidelines. Obtaining a detailed medical history for each subject in a smaller trial, for example, makes his or her individual response to a certain drug all the more revealing.

This strategy can only work, however, if a high percentage of patients with a rare disease are willing to join a clinical trial. And that’s where people like Peeper become invaluable. Thanks to the active global community she created, any clinical trial for an FOP drug now has hundreds of potential participants.

On one of my visits to Philadelphia, Kaplan took me to see Harry. We met in the pillared entryway of the College of Physicians of Philadelphia, a medical society founded in 1787. Kaplan was wearing a tie covered in skeletons. We descended a flight of stairs to the Mütter Museum, an eerie basement collection of medical specimens. We passed cabinets filled with conjoined twins, pieces of Albert Einstein’s brain, and a cadaver turned to soap. We walked up to a glass case, which a curator opened for us. Inside loomed a skeleton beyond imagining.

It belonged to Harry Eastlack, a man with fibrodysplasia ossificans progressiva who asked shortly before he died in 1973 that his body be donated to science. Harry stands with one leg bent back, as if preparing to kick a soccer ball, and the other hinged unnaturally forward; his arms hover in front of his body; his back and neck curve to one side, forcing his eye sockets to gaze at the floor. Before a typical skeleton goes on display, the bones have to be wired and bolted together. Eastlack’s skeleton needed almost no such help. It is a self-supporting scaffolding, its original structure overlain with thorns, plates, and strudel-like sheets.

“The first time I saw Harry, I stood here mesmerized,” Kaplan told me, shining a red laser on a ligament in Harry’s neck that had become a solid bar running from the back of his head to his shoulders. “I’m still learning from him.”

Thanks to Kaplan’s enduring fascination with her disease, Jeannie Peeper can now realistically imagine a time—perhaps even a few years from now—when people like her will take a pill that subdues their overactive bones. They might take it only after a flare-up, or they might take a daily preventative dose. In a best-case scenario, the medication could allow surgeons to work backwards, removing extra bones without the risk of triggering new ones.

At 54, with an advanced case of FOP, Peeper does not imagine that she’ll benefit from these breakthroughs. But she is optimistic that her younger friends will, and that one day, far in the future, second skeletons will exist only as medical curiosities on display. All that will remain of her reality will be Harry Eastlack, still keeping watch in Philadelphia, reminding us of the grotesque possibility stored away in our genomes.

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