Anti-follistatin like 1 antibodies are used in the immunodetection of the protein encoded by the FSTL1 gene. In humans, the canonical protein has a reported length of 308 amino acid residues and a mass of 35 kDa. It is known to be a secreted protein. Alternative splicing is reported to yield 2 different isoforms for this protein. It is notably expressed in synovial tissues from rheumatoid arthritis. It is reported to be a secreted glycoprotein that is involved in various physiological processes, such as angiogenesis, regulation of the immune response, cell proliferation and differentiation. Post-translational modifications have been described, including glycosylation. Other names for this target antigen include FSL1, MIR198, OCC-1, OCC1, tsc36, follistatin-related protein 1, follistatin-like protein 1, and FRP.