But the problem lies.......

The gene which causes ossification is normally deactivated after a fetus’ bones are formed in the womb—but in patients with FOP, the gene keeps working. Aberrant bone formation in patients with FOP occurs when injured connective tissue or muscle cells at the sites of injury or growth incorrectly express an enzyme for bone repair during apoptosis (self-regulated cell death), resulting in lymphocytes containing excess bone morphogenetic protein 4 (BMP4) provided during the immune system response. The bone that results occurs independently of the normal skeleton, forming its own discrete skeletal elements. These elements, however, can fuse with normal skeletal bone. 

Interestingly, the diaphragm, tongue, and extra-ocular muscles are spared in this process, as well as cardiac and smooth muscle. Since the incorrect enzyme remains unresolved within the immune response, the body continues providing the incorrect BMP4-containing lymphocytes—BMP4 is a protein which contributes to the development of the skeleton in the normal embryo.

Because the disease is so rare, the symptoms are often misdiagnosed as cancer or fibrosis—leads doctors to order biopsies, which can actually exacerbate the growth of these lumps.