Follistatin-Related Protein 1 (FSTL1) is a secreted protein that contains two EF-hand domains, one follistatin-like domain, one Kazal-like domain, and one VWFC domain. Its functional significance in physiological and pathological processes is not completely understood. However, FSTL1 is thought to modulate the action of some growth factors on cell proliferation and differentiation. FSTL1 maybe an autoantigen associated with rheumatoid arthritis.