The mRNA expression patterns of the ADAM species in synovial tissues could be classified into three groups: constitutive expression in both RA and OA samples (ADAM9, ADAM10 and ADAM17); negligible or no expression in RA or OA (ADAM8, ADAM20, ADAM21, ADAM28 and ADAM30); and more selective expression in RA than in OA (ADAM12 and ADAM15). Here, ADAM21 is linked to rheumatoid arthritis.