Reduced Intensity AlloTransplant For Osteopetrosis

Inclusion Criteria: * Patients eligible for transplantation under this protocol will be \<45 years of age, and will be diagnosed with severe osteopetrosis. This will be defined as having the following manifestations of the disease. * Bones that are uniformly markedly dense based on skeletal survey * No history that would suggest autosomal dominant inheritance * Evidence of hematologic changes that are attributed to the underlying disease, including the need for ongoing transfusions, OR * the presence of progressive anemia or thrombocytopenia, OR a white blood cell differential with a predominance of immature forms and evidence of extramedullary hematopoiesis, OR * persistence of serious infectious complications that are thought to be due to the abnormal architecture of the bone that are resistant to surgical and medical interventions. Exclusion Criteria: * Patients \>45 years of age * Evidence of hepatic failure * pulmonary dysfunction sufficient to substantially increase the risk of transplant * Renal dysfunction with glomerular filtration rate (GFR) \<30% of predicted. * Cardiac compromise sufficient to substantially increase the risk of transplantation * Severe, stable neurologic impairment. * Human immunodeficiency virus (HIV) positivity. * Pregnant or lactating females