Whilst the blockade of ALK2 signaling through ligand binding, inhibition of ALK2 kinase activity, or upregulation of the expression of ACVR1 at the transcriptional level have been explored in experimental models of fibrodysplasia ossificans progressiva (a rare heritable form of HO), retinoid use has off-target effects and the mechanistic justification for their use in post-traumatic HO is not established [14]. The gene discussed is ACVR1; the disease is fibrodysplasia ossificans progressiva.