BMP4 and fibrodysplasia ossificans progressiva: The median levels of serum activin A, BMP4 and BMP6 in healthy control subjects were 364.14 pg/mL, 450.39 pg/mL, and 55.36 pg/mL, respectively, while the levels of serum activin A, BMP4 and BMP6 in FOP patients were 434.05 pg/mL, 459.48 pg/mL, and 67.84 pg/mL, respectively.