Osteopetrosis is a rare genetic disease that causes extremely dense and fragile bones. This is because people with osteopetrosis do not have normal osteoclasts, which bones need to function properly. Healthy bones require properly functioning osteoblasts and osteoclasts. Osteoblasts are responsible for producing new bones, and osteoclasts are bone cells responsible for bone resorption, which is the breaking down of bones and providing space for new bone marrow to grow. An individual with osteopetrosis has osteoclasts that do not function properly, so their bones are unhealthy (Stocks, Wang, Thompson, Stocks, & Horwitz, 1998). There are three types of osteopetrosis, each varying in its severity. The first type is called malignant infantile osteopetrosis. This type is the most serious. The next type is called mild autosomal recessive or intermediate type. This type is not as serious as the malignant form, but can still present many complications and symptoms. The last type is the autosomal dominant type or the adult type. This is the mildest version of the disease; in fact many of this type are asymptomatic ("Osteopetrosis", 2008). Malignant infantile osteopetrosis is autosomal recessive, present from birth, and is the most severe form of osteopetrosis. Because this type is recessive, the disease does not manifest itself unless the person receives the same defective gene from the mother and father. If both parents carry the recessive gene, there is a 25% chance that a child will be infected with the malignant form of osteopetrosis (“Osteopetrosis”, 2008). The gene responsible for this disease is located "on the long arm of chromosome 11 (llq12-q13)" ("Osteopetrosis, p. 4). Malignant......in the center of the sheet......detected November 27 2011, from http://aafp.orgDozier, T., Duncan, I., Klein, A., Lambert, P., & Key, L.J. (2005, April 26). Otological manifestations of malignant osteopetrosis. National Center for Biotechnology Information. Retrieved November 27, 2011, from http://ncbi.nlm.nih.govKolb, MD, AE (n.d.). What is osteopetrosis?. Mason Shaffer Foundation || Save a life... Donate cord blood. Retrieved November 27, 2011, from http://masonshafferfoundation.orgOsteopetrosis. (2008, May 4). Osteopetrosis. Retrieved November 29, 2011, from http://raredisease.orgStocks, MD, MS, Wang, MD, WC, Thompson, MD, JW, Stocks, MD, MC, & Horwitz, MD, EM (n.d.). Malignant infantile osteopetrosis. Archives of Otolaryngology—Head & Neck Surgery, a monthly peer-reviewed medical journal published by the AMA. Retrieved November 27, 2011, from http://archoto.com