Both Follistatin (FST) and Follistatin-like-3 (FSTL3) irreversibly bind Activin dimers and prevent Activin from interacting with its receptor (reviewed in Schneyer et al. 2004 ,Xia and Schneyer 2009). Though functionally similar in vitro, FST and FSTL3 do not function identically in vivo. Mice lacking FST die shortly after birth due to defects in muscle and bone (Matzuk et al. 1995); mice lacking FSTL3 are viable but have altered glucose metabolism (Mukherjee et al. 2007).