Jansen type metaphyseal chondrodysplasia is an extremely rare progressive disorder in which portions of the bones of the arms and legs develop abnormally with unusual cartilage formations and subsequent abnormal bone formation at the large (bulbous) end portions (metaphyses) of these long bones (metaphyseal chondrodysplasia). As a result, affected individuals exhibit unusually short arms and legs and short stature (short-limbed dwarfism), findings that typically become apparent during early childhood. Abnormal cartilage and bone development may also affect other bones of the body, particularly those of the hands and feet (i.e., metacarpals and metatarsals). Infants with Jansen type metaphyseal chondrodysplasia may also have characteristic facial abnormalities and additional skeletal malformations. During childhood, affected individuals may begin to exhibit progressive stiffening and swelling of many joints and/or an unusual “waddling gait” and squatting stance. In addition, affected adults may eventually develop abnormally hardened (sclerotic) bones especially in the back of the head (cranial bones), which, in some cases, may lead to blindness and/or deafness. In addition, in some cases, affected individuals have abnormally high levels of calcium in the blood (hypercalcemia). The range and severity of symptoms may vary from case to case. Most cases of Jansen type metaphyseal chondrodysplasia occur randomly as the result of a spontaneous genetic change (i.e., new genetic mutation)