HTRA1 and osteoarthritis: HtrA1, first described by Zumbrunn and Trueb (1), is expressed in different normal human tissues (2) and appears to be involved in several physiological processes, through inhibition of extracellular protein transforming growth factor β (TGF-β) signaling in vivo and in vitro (3), as well as in the pathogenesis of diseases such as amyloid degeneration, senile macular degeneration, Alzheimer’s disease, osteoarthritis and pre-eclampsia (4–7).