ProMMP9 binds Tissue Inhibitor of Metalloproteinases 1 (TIMP1) (Wilheim et al. 1989). By homology with the binding of TIMP2 to the MMP2 hemopexin domain, TIMP1 is thought to binds via its C domain to the MMP9 hemopexin domain. This is a two-way potential regulatory mechanism as the interaction does not prevent the free N-terminal inhibitory domain of TIMP1 from inhibiting other MMPs (Murthpy et al. 1991) while it may prevent the activation of proMMP9.