CTSK and osteopetrosis: Double deletion of Mmp9 and Ctsk causes increased bone mass and severe osteopetrosis in mice. To determine whether Ctsk may compensate for the loss of Mmp9 in Mmp9-/- mice during skeletal development, we generated a Ctsk-/-/Mmp9-/- DKO mouse model by crossing Ctsk-/- and Mmp9-/- mice.