IHC indicated profuse cytoplasmic CYP1B1 expression in fibroblastic and large mononuclear cells of lining and sublining areas in RA and OA synovial tissues (Fig. 5a); evaluation showed that the fractional area immunostained by CYP1B1 was larger in OA than in RA tissues (p = 0.002). Here, CYP1B1 is linked to rheumatoid arthritis.