2B0U:
reference: The structure of the follistatin:activin complex reveals antagonism of both type I and type II receptor binding., Thompson TB, Lerch TF, Cook RW, Woodruff TK, Jardetzky TS, Dev Cell 2005 Oct;9(4):535-43. PMID: 16198295

This OCA Structure page
uses Jmol, developed by the Jmol Development Team (documentation).