The purpose of this study is to establish a registry of all children with severe, malignant osteopetrosis who are treated with Actimmune (IFN-g 1b or Interferon gamma-1b) to monitor the effects of IFN-g 1b on preventing progression of this disease and to follow the safety of patients receiving it on a long-term basis. In addition, evaluation of the possible effect of Actimmune therapy on the humoral response to normal childhood vaccinations in this same patient population will be examined.Interferon gamma is a substance that the body makes naturally.

It is made by white blood cells and appears to be involved in regulating the body's ability to fight off infection. Actimmune is a synthetic form of Interferon gamma which is similar to that normally made by white blood cells.

IFN-g 1b (Actimmune®) is currently approved by the United States Food and Drug Administration (FDA) for the treatment of patients with chronic granulomatous disease (CGD) to reduce the frequency and severity of serious infections. It is also approved in patients with severe, malignant osteopetrosis to delay the time to disease progression. In research trials, IFN-g 1b has been given to over 2,000 patients in diseases such as CGD, osteopetrosis, atopic dermatitis, pulmonary fibrosis, atypical mycobacteria and various cancers.

• Male or female
• Diagnosis of severe, malignant osteopetrosis
• Currently receiving or planning to initiate therapy with Actimmune (Interferon gamma-1b)
• Willing to attend follow-up appointments every 6 months following enrollment into the study, if clinically indicated