The positron emission tomography (PET) tracer <i>N</i>-succinimidyl 4-[<sup>18</sup>F]-fluorobenzoate-conjugated folate ([<sup>18</sup>F]-SFB-FOL) effectively targets FR-β-positive macrophages in rheumatoid arthritis. This evidence concerns the gene FOLR2 and rheumatoid arthritis.