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Familial expansile osteolysis is a rare bone dysplasia which is transmitted as an autosomal dominant trait in a large kindred in Northern Ireland. The gene which causes the disease shows tight linkage with several polymorphic markers on chromosome 18q with a maximum lod score of 11.53 at a recombination fraction of 0.00 with D18S64. The gene is flanked by D18S35 and D18S61 and is located at chromosome 18q21.1-q22. Mapping a new locus for a gene involved in regulation of bone metabolism may also have implications in the study of Paget's disease of bone which is a common related bone dysplasia.

Familial expansile osteolysis (FEO) is a unique bone dysplasia, which has, over five generations, affected 42 members of a Northern Ireland family. The disease follows a classic autosomal dominant pattern of inheritance. The condition is distinct enough in its clinical features and natural history to be recognized as a new and unique disease. There are both general and focal skeletal changes, the latter having a predominantly peripheral distribution and an onset from the second decade. Progressive osteoclastic resorption accompanied by medullary expansion leads to severe and painful disabling deformities with a tendency to pathologic fracture. Most affected members of the family have an associated early-onset deafness and loss of dentition as a result of unique middle ear and dental abnormalities. The serum alkaline phosphatase and urinary hydroxyproline are elevated to a variable degree, whereas other biochemical indices are normal. The response of the disease to a therapeutic trial using parenteral dichloro-methylene-diphosphonate (dichloro-MDP) produced an initial rapid biochemical response, which was not sustained.