Recently, Watanabe et al. showed that monocytes from patients with GCA produce high amounts of matrix metalloproteinase 9 (MMP-9), which allows them to digest the basement membrane of vasa vasorum capillaries and enter the adventitial tissue, and thus exerting tissue-invading abilities and at the same time facilitating the invasion of other inflammatory cell populations (30). This evidence concerns the gene MMP9 and temporal arteritis.