This website is intended for U.S. residents only.
This website is intended for U.S. residents only.

What is FOP?

Fibrodysplasia Ossificans Progressiva or FOP is an ultra-rare genetic disorder that causes bone to form where it should not – outside of the skeleton and in the soft and connective tissues such as in the muscles, tendons and ligaments.1,2

The process of bone forming outside of the skeleton is called Heterotopic Ossification or HO.3 Over time HO leads to ribbons, sheets and plates of extra bone forming in places throughout the body where bone should not be.1

what is fop

*This is an artistic portrayal.

Once formed, this extra bone does not go away and can lead to joint stiffness. Where it forms across the joints it can progressively restrict normal movement, potentially causing locked joints (such as locked arms or a locked back).1,4

Extra bone formation in people living with FOP can be triggered by knocks and falls and certain medical procedures, such as taking a biopsy or having an intramuscular injection.5

It is important to get a diagnosis of FOP and be aware of how to avoid flare-ups of the condition.

Learn more about FOP flare-ups

What Causes FOP?

FOP is caused by having a faulty copy of a gene. Scientists discovered that the gene responsible is Activin-Like Kinase 2 or ACtiVin Receptor type-1 (ALK2/ACVR1).

In people without FOP, this gene produces a normal protein. This protein interacts with specific bone-forming proteins called Bone Morphogenetic Proteins (BMPs). And through this interaction, normal bone is formed.

However, in people with FOP, this faulty gene leads to a malformed, overactive protein. This overactive protein leads to the formation of extra bone.


FOP toes

A person who is living with FOP would need to have either inherited a faulty copy of the ALK2/ACVR1 or ‘FOP gene’ from a parent with the condition or had a spontaneous mutation occur in the gene while developing in the womb. Most cases of FOP occur in people with no family history of the disease.7

FOP is a congenital genetic condition which means that it is present at birth. However, the diagnosis will often not be made until much later in childhood when a flare-up arouses suspicion.8 There is one clear warning sign of FOP, however, and that is being born with turned-in great toes that may look like bunions (hallux valgus) but they are not.

Learn more about FOP signs and symptoms

How is FOP managed?

There is currently no cure for FOP and management is primarily focused on:9

  • careful avoidance of activities or procedures that might cause injury or harm and thus extra bone to form during or after a flare-up
  • medication to manage FOP symptoms such as provide relief for the pain and inflammation that occurs during flare-ups
  • optimizing existing physical function

Learn more about preventing FOP flare-ups


Researchers discovered the gene responsible for extra bone formation in FOP in 20066

FOP Facts

FOP is ultra-rare

Only around one or two people per million have the condition.1 The number of people living with FOP at any one time varies from country to country.10


Extra bone due to FOP is normal bone, it’s just in the wrong place

Bone that forms following a FOP flare-up is the same bone that the body makes normally. This bone is just formed in the wrong place, such as soft connective tissue like ligaments and tendons or certain muscles. Over time this bone may start to build up and restrict movement across the joints and cause problems with mobility.1,4

FOP is caused by a defect in a single gene

FOP is an ultra-rare genetic disorder that is present from birth.7 It is caused by a defect in the ALK2/ACVR1 gene which is involved in the normal formation of bone.7


FOP may be misdiagnosed at first

Just over half of all people living with FOP, were diagnosed with a different disease before they get an accurate diagnosis. It is commonly misdiagnosed as a disease with similar symptoms, such as cancer, other tumors, bunions or other rare diseases.11,12

FOP diagnosis can be confirmed by a genetic test

A diagnosis of FOP can be made based on clinical signs and symptoms such as having turned-in great toes and recurring flare-ups. There is also a genetic test for the FOP gene that can confirm a diagnosis of FOP.9,13,14

FOP can affect anyone

FOP can affect girls as well as boys regardless of their race, ethnicity or country of origin.5 And while FOP can be inherited, the majority of patients with FOP experienced a spontaneous genetic mutation.

FOP can’t be reversed

The extra bone that is formed does not go away.1,3 This means it can build up over time and make it difficult for some people living with FOP to move and perform their normal everyday tasks. Recognizing and avoiding certain activities to help prevent FOP flare-ups is important to try to limit the formation of additional bone.1,4

flare up

Doctors and researchers are advancing the science

Since the FOP gene was discovered, doctors and researchers have been working hard to better understand this condition and how it affects patients. 7,13,14

FOP management requires a team of professionals

From providing medical care to social support, many different healthcare professionals work together to provide the best care for people living with FOP.13,14


There is a growing FOP community

In 2019, a third of the world’s FOP population was part of the International Fibrodysplasia Ossificans Progressiva Association (IFOPA). IFOPA is an international non-profit organization that provides support to people living with FOP and their families through education and support programs.15

Join the FOP Community

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Please always consult a healthcare professional if you require healthcare advice or if you have any specific concerns regarding Fibrodysplasia Ossificans Progressiva. The information provided here is not intended to replace professional advice. This website has been developed by Ipsen in collaboration with those living with Fibrodysplasia Ossificans Progressiva and the healthcare professionals who care for them. Ipsen would like to thank everyone for their valuable insights and stories. All names used on this website are not necessarily real names. Visit our website for more information about us, or to contact us directly. Website design and development by Kanga Health Ltd. Website reference NON-US-001996 December 2020