The purpose of this research is to explore what we believe may be a safer and more effective means of performing stem cell transplantation in patients with Osteopetrosis, using chemotherapy and radiation designed to bring about engraftment and lessen transplant mortality. Prior multi-institutional data in past studies found that approximately 30% of Osteopetrosis patients do not engraft. Therefore, in this study, we utilize a reduced intensity design of pre-transplant drugs to try to make transplants safer for this disease, as well as to provide a second infusion of stem cells in patients with matched related or unrelated donors.