Fibrodysplasia ossificans progressiva (FOP) is a rare, severely disabling disease characterized by painful, recurrent episodes of soft tissue swelling (flare-ups) that result in abnormal bone formation in muscles, tendons, and ligaments. Flare-ups begin early in life and may occur spontaneously or after soft tissue trauma, vaccinations, or influenza infections. Recurrent flare-ups progressively restrict movement by locking joints leading to cumulative loss of function and disability. Mouse models of FOP have demonstrated the ability of retinoic acid receptor (RAR) gamma agonists to prevent heterotopic ossification (HO) following injury. The purpose of the study is to evaluate whether palovarotene, an RAR gamma agonist, will prevent HO during and following a flare-up in subjects with FOP.