In addition, both primary connective tissue progenitor cells and induced human pluripotent stem cell-derived endothelial cells from FOP patients showed increased SMAD1/5/8 signalling upon BMP4 stimulation [16, 17], and SMAD1/5/8 is the downstream protein complex of ACVR1R206H. Here, SMAD1 is linked to fibrodysplasia ossificans progressiva.