Mice deficient for MMP-13 were protected from antibody-induced arthritis. Mice deficient for matrix metalloproteinase-13 (MMP-13–/–; n = 21) and wild-type mice (n = 20) were treated with 200 μl K/BxN sera, and arthritis was evaluated by measuring (A) the ankle thickness and (B) the clinical index. There was a significant difference in ankle thickness and clinical index scores between the two groups. Values represent the mean ± standard deviation of ankles/time points, which were compared using the Student t test. *P <0.05, **P <0.01. KO, knockout.