In these mice, application with recombinant M-CSF or generation of soluble M-CSF from osteoblasts increases the number of osteoclasts and alters the phenotypes of osteopetrosis, indicating that M-CSF is needed for the generation of osteoclasts in youthful mice, but does not rule out the possibility of compensating systems that are not relying on M-CSF (Kim J.-M. Here, CSF1 is linked to osteopetrosis.