“Fibrodysplasia ossificans progressiva (FOP)  can be caused by a genetic condition where people are born with bunions and their body’s muscle tissue and connective tissues, like tendons and ligaments, turn into bone on the outside of their skeleton. This condition restricts movement and can cause a loss of mobility over time in people diagnosed with the condition.

Fibrodysplasia ossificans progressiva can also affect anyone because it’s most often the result of a new genetic mutation that wasn’t inherited from a parent. Genetic mutations are unpredictable. They occur during fertilization due to an error when cells divide and replicate but their can be outside factors as causes also.”

Cleveland Clinic (Fibrodysplasia Ossificans Progressiva: Causes, Symptoms, Treatment, Outlook)