Day 4 mature osteoclasts were pre-treated with a farnesyl transferase inhibitor to block Ras activation or vehicle control for seven hours. Cells were serum starved and treated with 25 ng/ml M-CSF as indicated in the presence of inhibitor or vehicle control. Western blotting for phospho-PAK1Thr423, total PAK1/3, phospho-MEK1/2Ser217/221, total MEK1/2, phospho-ERKThr202/Tyr204, total ERK1/2 and tubulin was then performed.