Shi et al.(37) investigated early changes in the concentration of MMP-2, -9 and tissue inhibitor of metalloproteinase (TIMP-1 ) in a rat model of brain injury combined with trauma-induced heterotopic ossification and showed that MMP-9, but not MMP-2, contributed to the remodeling and calcification of the ECM, resulting in the induction of osteoblast precursor cells in heterotopic ossification. Here, TIMP1 is linked to brain injury.