Follistatin-related protein 1 (FSTL1) is also known as Follistatin-like protein 1 (FRP). FSTL1 is a secreted protein that contains two EF-hand domains, one follistatin-like domain, one Kazal-like domain and one VWFC domain. FSTL1 is overexpressed in synovial tissues from rheumatoid arthritis. Follistatin-like protein 1 / FSTL1 may modulate the action of some growth factors on cell proliferation and differentiation. FSTL1 binds heparin.