While studying human MMP-2 deficiency involving a cohort of MMP-2 deficient patients, rheumatoid arthritis patients and healthy controls (Table S1), we made a serendipitous observation where we identified a control blood donor (denoted herein as abnormal) whose serum proMMP-2 demonstrated an unusual lack of binding to gelatin. Here, MMP2 is linked to rheumatoid arthritis.