The 177R4 monoclonal antibody reacts with mouse Fibrinogen-like protein 1 (FGL-1). FGL-1 is a member of the fibrinogen family of proteins. Under normal physiological conditions, FGL-1 is primarily secreted from hepatocytes and contributes to mitogenic and metabolic functions. FGL-1 is produced at high levels in various tumors including lung cancer and melanoma. High FGL-1 expression is associated with resistance to anti-PD-1/PD-L1 therapy and poor prognosis of cancer patients. Recently, FGL-1 has been identified as a major inhibitory ligand for LAG-3, a receptor that negatively regulates the proliferation, activation, and effector function of T cells. In murine cancer models, the 177R4 clone has been shown to block FGL-1 binding to LAG-3 resulting in boosted T cell activity and slowed tumor growth.