The osteoclast-not always guilty.

Osteopetrosis is usually regarded as the disease of nonfunctioning osteoclasts, with a consequent accumulation of osseocartilaginous material in the bone marrow. A recent report shows that, in some patients with the rare autosomal recessive form of the disease, the osteoclast defect may be non-cell autonomous, in this case due to a mutation in RANKL, the protein that normally controls osteoclast formation and activity.