What is FOP?

What is Fibrodysplasia Ossificans Progressiva (FOP)?

Malformation of the great toes

One of the rarest, most disabling genetic conditions known to medicine, FOP causes bone to form in muscles, tendons, ligaments and other connective tissues. Bridges of extra bone develop across joints, progressively restricting movement and forming a second skeleton that imprisons the body in bone. There are no other known examples in medicine of one normal organ system turning into another.

Demographics of FOP:

  • Rare, progressive genetic disorder that has an estimated prevalence of 0.88 per 1 million US residents, or approximately a prevalence of 1 in 1 million.* 
  • No ethnic, racial, or gender patterns
  • Approximately 900 confirmed cases worldwide
Flare-ups on the back of a young child

Clinical Characteristics of FOP:

  • Malformation of the great toes is visible at birth
  • Flare-ups that worsen the condition occur spontaneously or following viral illnesses or physical trauma to the muscle such as: intramuscular childhood immunizations, falls, surgery, biopsy
  • Rogue bone growth progressively restricts movement
  • Often misdiagnosed as cancer (learn more about misdiagnosis)
  • The exact rate of progression is unpredictable, although there appears to be a pattern to the progression (e.g., upper body in childhood and lower body in adolescence)


Research and Clinical Studies & Trials: 

Learn the science behind FOP:


* Pignolo RJ, Hsiao EC, Baujat G, Lapidus D, Sherman A, Kaplan FS. Prevalence of fibrodysplasia ossificans progressiva (FOP) in the United States: estimate from three treatment centers and a patient organization. Orphanet J Rare Dis. 2021;16(1):350. Published 2021 Aug 5. doi:10.1186/s13023-021-01983-2

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