Collagenase activity in diseased periodontal tissues is mainly contributed by MMP‐8 instead of other collagenases MMP‐1 and ‐13, and especially active MMP‐8 but not total MMP‐8 is associated with and reflects periods of active connective tissue destruction and a clinical diagnosis of periodontitis (Romanelli et al., 1999). The gene discussed is MMP8; the disease is periodontitis.