FSTL4 (Follistatin-like protein 4; also FSL4 SPIG1 and D/Bsp120I-1) is a likely secreted 90 kDa (predicted) member of the follistatin gene family of TGF-beta superfamily inhibitors. It is widely expressed being found in neurons (retinal ganglion and cerebellar Purkinje cells) cardiac muscle cells smooth muscle cells and intestinal epithelium. Mature mouse FSTL4 is 819 amino acids (aa) in length (aa 23-841). It contains one kazal-like domain (aa 80-134) an EF-hand motif (aa 173-208) and two Ig-like domains (aa 250-336 and 340-425). There are two potential splice forms one that shows an alternative start site at Met315 and a second that possesses a His residue substitution for aa 518-841. Full-length mature mouse FSTL4 shares 95% and 84% aa identity with rat and human FSTL4 respectively.