FCMR encodes a high-affinity Fc receptor specific for immunoglobulin M (IgM) that plays critical roles in immune homeostasis and B cell regulation 1. The receptor binds both secreted and membrane-bound IgM, with binding occurring through the Cμ4 domain of IgM in a glycan-independent manner 2. FCMR primarily regulates IgM transport and homeostasis, enabling exocytosis of membrane-bound IgM and endocytosis of IgM-antigen complexes for lysosomal degradation 1. The receptor exhibits species-specific expression patterns, being found on B, NK, and T cells in humans, while predominantly expressed on B cells in mice 3. FCMR mediates costimulatory signaling and contributes to B cell development and primary immune responses, likely limiting tonic IgM signaling to self-antigens for proper negative selection of autoreactive B cells 1. Cell-surface expression of FCMR is regulated by cell density rather than IgM abundance, with higher expression observed in circulating lymphocytes than previously appreciated 3. The receptor has been identified as a diagnostic biomarker in certain pathological conditions, including idiopathic pulmonary fibrosis 4, and its downregulation has been implicated in cancer progression 5.