FCRL6 is a transmembrane glycoprotein that functions as an MHC class II receptor expressed on mature cytotoxic lymphocytes, particularly CD8+ T cells and NK cells 1. Unlike other Fc receptor-like family members, FCRL6 does not bind immunoglobulin and does not directly promote cytokine production or cytotoxic granule release when stimulated alone; instead, it recruits inhibitory phosphatases (SHP-1, SHP-2, SHIP-1, SHIP-2) and adaptor proteins through phosphorylated tyrosines, suggesting an inhibitory role in effector lymphocyte function 2. FCRL6 expression increases with lymphocyte differentiation and distinguishes antigen-experienced cytotoxic cells in peripheral blood and spleen. In disease contexts, FCRL6 has emerged as a biomarker with prognostic significance. The receptor is upregulated in patients with B-cell chr1 lymphocytic leukemia, where it marks expanded cytotoxic populations 1, and in melanoma, breast, and lung cancer patients who relapsed after PD-1 blockade, suggesting a potential mechanistic role in adaptive immune evasion to checkpoint inhibitor therapy 3. High FCRL6 expression serves as a biomarker for immune activation in people living with HIV with CMV seropositivity 4, and differential FCRL6 levels associate with distinct serum proteome profiles in Crohn's disease, with lower levels marking isolated ileal ulcers 5. These findings position FCRL6 as both a biomarker for immune dysregulation and a potential immunotherapeutic target in cancer and chr1 inflammatory conditions.