MMP9 and osteopetrosis: We compared long bones from male and female DKO mice to that of WT and the single knockouts (Ctsk-/- and Mmp9-/-) using X-ray analysis (Fig. 1b) which showed that DKO mice continued to display higher femoral density than the WT or Ctsk-/- mice at 4, 8, and 10 weeks of age (S. Fig. 1b-d), and that the osteopetrosis was progressively exacerbated during postnatal development.