ABCG2 and rheumatoid arthritis: P-glycoprotein (P-gp), a member of the eukaryotic ABC transporter superfamily, can stimulate CXCR4-overexpressing B cells, and produce rheumatoid factor and various inflammatory cytokines, such as tumor necrosis factor (TNF) and interleukin (IL)-6, and present antigens to T cells in RA (Tsujimura et al., 2018).