||Follistatin-like 1, also known as FSTL1, is an activin-binding protein with similarity to follistatin. It contains an FS module, a follistatin-like sequence containing ten conserved cysteine residues, two EF-hand domains, a Kazal-like domain and a VWFC domain. This heparin binding protein is involved in the BMP signaling pathway, and may modulate the action of some growth factors on cell proliferation and differentiation. FSTL1 is thus suggested as a regulator of early mesoderm patterning, somitogenesis, myogenesis and neural development. FSTL1 is ubiquitously expressed in the majority of tissues in adult, with strongly expressed level in the mesenchymal component and excluded from the epithelium. In addition, this protein is thought to be an autoantigen associated with rheumatoid arthritis.