Tanaka and his team have been investigating the cloning of follistatin-related protein (FSTL1) as a novel auto-antigen in systemic rheumatic disease and found it to have an ameliorative effect on joint inflammation in mouse models. FSTL1 is a member of the follistatin protein family that has been found in immunological studies to modify joint inflammation in arthritic disease and modulate allograft tolerance. However, FSTL1’s main physiological function is unknown. Tanaka and his team cloned four FSTL1-associated proteins using a two-hybrid cloning method: disco-interacting protein 2 homolog A from Drosophila (DIP2A) and confirmed that DIP2A could be a cell-surface receptor protein and mediate a FOS down-regulation signal of FSTL1. They also found in molecular interaction analyses using Biacore, that FSTL1 bound to DIP2A and CD14, and also with proteins of the TGF-b super-family. FSTL1 binding to DIP2A was blocked by CD14, follistatin, activin and BMP-2. FSTL1 blocked the ligand-receptor binding of activin and BMP-2, but integrated itself with that of BMP-4. This multi-specific binding may reflect the broad physiological activity of FSTL1.