ACVR1 and fibrodysplasia ossificans progressiva: This lack of a disease-specific effect of Activin-A on osteoclasts could suggest that other Activin-A receptors on the osteoclast precursors overruled the mode of action of Activin-A in binding to ACVR1, albeit that ACVR1 is transcribed in osteoclasts and that the mutated form was transcribed at a ninefold higher level compared with the non-mutated form in osteoclasts derived from FOP patients [14].