(A) Representative microCT images of the hind limbs of uninjured (n = 3) and injured gHO mice treated with vehicle (n = 4) or ACVR1 kinase activity inhibitor (n = 5) at 28 dpi. White arrows indicate HO. Scale bars: 1 mm. (B and C) Quantification showing HO volume (B) and bone mineral content (C) in the gHO mice treated with vehicle or ACVR1 inhibitor. (D) Experimental timeline for the coculture of FAPs with Mrep. FAPs, non-FAPs, Mrep, and other cells (the remaining CD45⁺ cells) were sorted from muscles of gHO mice at 1 dpi. Anti–activin A antibody (1 mg/mL) was used to neutralize activin A. (E) Alizarin red S staining of the coculture experiment in D. Representative data from 3 independent experiments are shown. (F and G) Representative microCT images (F) and quantification (G) of HO in Acvr1^Q207D-induced HO of Inhba^fl/fl mice (n = 26) and Inhba^fl/fl LysM-Cre mice (n = 17) at 28 dpi. White arrows indicate HO. Scale bars: 1 mm. (H) RT-qPCR results showing the relative Inhba expression in the muscles at 1 dpi from the mice treated with DMSO (n = 5) or TAK-242 (n = 5). (I–K) Representative microCT images (I and J) and quantification (K) of HO in gHO mice treated with vehicle (n = 10), TAK-242 (n = 7), and clodronate (n = 7) at 28 dpi. White arrows indicate HO. Scale bars: 1 mm. The P values were calculated using unpaired 2-tailed t test (B, C, G, and H) and 1-way ANOVA with Tukey’s multiple-comparison test (K). A P value < 0.05 was considered significant. Data are shown as the mean ± SEM, and symbols represent individual mice.