Osteoclasts are large multinucleated cells responsible for bone resorption. Bone resorption is dependent on the liberation of calcium by acid and protease destruction of the bone matrix by proteinases. The key proteinase produced by the osteoclast is cathepsin K. Targeted knock-down of cathepsin K was performed using small inhibitory RNA (siRNA). siRNA is a method that introduces short double-stranded RNA molecules that instruct the RNA-induced silencing complex (RISC) to degrade mRNA species complementary to the siRNA. Transfection of siRNA by lipid cations allows for short-term inhibition of expression of the targeted gene. We show that transfection of primary human osteoclasts with siRNA to cathepsin K reduces expression by 60% and significantly inhibits bone resorption with a reduction of both resorption pit numbers (P = 0.018) and resorbed area (P = 0.013). We also show that FuGENE 6 is an effective lipid transfection reagent with which to transfect primary human osteoclasts, that does not produce off-target effects. 頲005 Wiley-Liss, Inc.