Fc mu receptor (FCMR) (FCMR)

The Fc mu receptor (FCMR), also known as TOSO or FAIM3, is a type I transmembrane protein and a member of the immunoglobulin superfamily that specifically binds the Fc region of IgM antibodies. In humans, it is predominantly expressed on B, T, and NK cells, where it regulates B-cell development, activation, and immune tolerance. FCMR is notably overexpressed in chronic lymphocytic leukemia (CLL), making it a promising therapeutic target for this malignancy. Upon binding to IgM, the receptor undergoes rapid internalization, a property being exploited for the development of IgM-based antibody-drug conjugates (ADCs) like Fcμ-MMAF. Additionally, experimental CAR-T cell therapies targeting FCMR have shown potential in selectively eliminating CLL cells while minimizing impact on healthy B cells. Although originally named for its suspected role in inhibiting Fas-mediated apoptosis, its primary physiological function is now understood to be the regulation of lymphocyte homeostasis and IgM-mediated immune responses.

Other names

TOSOFAIM3Fas apoptotic inhibitory molecule 3Regulator of Fas-induced apoptosis TosoFcmuRIgM Fc receptor

Targeted drug delivery via receptor-mediated internalization and CAR-T cell-mediated cytotoxicity.