Our interest in M-CSF expression in osteocytes stems from the findings of previous studies which noted that TNF-α increases the pool of osteoclast precursors available due to an increase in M-CSF production in stromal cells (11), and that M-CSF levels are increased in the synovial fluid around loose joint prosthesis (37) and in the serum of patients with rheumatoid arthritis (38), both of which exhibit a marked increase in TNF-α expression. This evidence concerns the gene CSF1 and rheumatoid arthritis.