- Cathepsin K, but not cathepsins B, L, or S, is abundantly expressed in human osteoclasts. Drake, F.H., Dodds, R.A., James, I.E., Connor, J.R., Debouck, C., Richardson, S., Lee-Rykaczewski, E., Coleman, L., Rieman, D., Barthlow, R., Hastings, G., Gowen, M. J. Biol. Chem. (1996)