Lysyl oxidase (LOX) is secreted to the extracellular space in an inactive, proenzyme form (proLOX). This is proteolytically cleaved between Gly168 and Asp169 generating the mature 32-kDa enzyme. The activating proteinase is Bone morphogenetic protein 1 (BMP1), also called Procollagen C-proteinase (Cronshaw et al. 1995, Panchenko et al. 1996). Other extracellular proteases, including the BMP1 variant mammalian tolloid, tolloid-like (TLL) 1 and TLL2 proteases cleave proLOX at the correct physiological site but with lower efficiency (Uzel et al. 2001).