INHBE and fibrodysplasia ossificans progressiva: FOP mice (Hatsell et al., 2015) and patient-derived induced pluripotent stem cells (Hino et al., 2015) were instrumental in the discovery of the fundamental and unexpected role of activin ligands in FOP pathogenesis, and antibody-based activin inhibition has emerged as a leading candidate therapeutic approach (Hatsell et al., 2015; Lees-Shepard et al., 2018; Upadhyay et al., 2017) that is now being evaluated in clinical trials.