IL34 and rheumatoid arthritis: The plasma concentration of IL-34 in synovial fluid and serum is low (Nandi et al., 2012; Tian et al., 2013), but in rheumatoid arthritis, the content of IL-34 is dramatically increased, hence it's level is a useful biomarker for predicting the progression of rheumatoid arthritis (Ding et al., 2015; Liu et al., 2018; Ge et al., 2019).