Double immunofluorescent labelling showed variable CYP1B1 expression by CD68+ lining macrophages in OA and RA synovia, although the frequency of CD68+ cells with very low or undetectable CYP1B1 staining appeared to be higher in RA than in OA samples (Supplementary Fig. S6); CYP1B1 was also detected in other CD68− cell types. Here, CD68 is linked to rheumatoid arthritis.