Although whether ECs contribute to ectopic bone formation in FOP patients remains controversial, we have recently demonstrated how circulating ECs isolated from FOP donors exhibit enhanced EndMT and osteogenic differentiation in vitro, which was used as a functional readout to identify novel small molecules targeting ALK2 (Sánchez-Duffhues et al., 2019b). The gene discussed is ACVR1; the disease is fibrodysplasia ossificans progressiva.