Immuno-histochemical and immuno-fluorescent analysis of FRβ expression in RA synovial biopsies showed marked expression of double CD68 and FRβ-positive macrophages in both lining and sublining of RA synovial tissue (representative example shown in Figure 3A) (25), thus encouraging further clinical evaluation of the [18F]fluoro-PEG-folate PET tracer. Here, CD68 is linked to rheumatoid arthritis.