Researchers worked with an antibody discovered by Alexion that interferes with the specific cell-surface receptor involved in FOP called activin A receptor type 1 (ACVR1). Their hypothesis was that if they blocked ACVR1, the abnormal bone growth would stop. However, it exacerbated the problem.

"The unexpected result: injecting the antibody into FOP mice caused a dramatic increase in inappropriate bone formation," said David Goldhamer, professor of molecular and cell biology at the University of Connecticut.

While researchers at Regeneron Pharmaceuticals used different antibodies and a different strain of FOP mice, they got the same adverse result. Neither research teams know why the respective antibodies dramatically worsened the disease in mice.