cell type responsible for activin a dependent ectopic bone formation in both the muscles and tendons of mice bearing the fop causing acvr1(r206h) mutation. in december 2019, ipsen issued a partial clinical hold for people under the age of 14, due to reports of early fusion of growth plates. recently, as of 2021, a potential therapeutic candidate, saracatinib, is in phase iii clinical trials as a potent heterotopic ossification inhibitor in wild-type and acvr1 mutant mice.