Yasmine et al. [41] observed the involvement of 5-HT in wild-type and Tph1−/− collagen-induced arthritis mice, and found that peripheral 5-HT levels in these mice were significantly reduced, and 5-HT could play an important role in RA by regulating Th17/Treg cell balance and osteoclastogenesis. Here, HTR5A is linked to rheumatoid arthritis.