Follistatin-like 1 (FSTL1) is a secreted glycoprotein that has been grouped into the follistatin family of proteins. FSTL1 is composed of a follistatin domain and two non-functional calcium-binding motifs. It was originally cloned as a TGFβ1 inducible factor but subsequently shown to regulate diverse developmental pathways and tissue homeostasis. Ablation of the FSTL1 gene in the mouse results in several structural developmental defects and neonatal lethality due to respiratory failure. FSTL1 suppresses BMP signaling, but the precise mechanism of its action has not been elucidated. FSTL1 is expressed in the human placenta, mainly in extravillous trophoblasts.