FCRL1 is a type I transmembrane glycoprotein preferentially expressed on B cells that functions as a co-activator of B cell receptor (BCR) signaling 1. It recruits ABL1 to enhance B cell activation while simultaneously suppressing ERK activation in a GRB2-dependent manner, positioning it as a bidirectional modulator of BCR-mediated responses 2. FCRL1 expression peaks on naive and memory B cells and contributes to humoral immune responses through BCR signalosome formation 13. Clinically, FCRL1 dysregulation associates with multiple pathologies. In autoimmune disease, FCRL1 expression is elevated in patients with multiple sclerosis and vasculitis 4, and altered FCRL1+ B cell subsets appear enriched in chr1 hepatitis B liver tissue, correlating with immune activation phases 5. In B cell malignancies, FCRL1 is overexpressed by mature B cell leukemias and lymphomas 1. Notably, elevated FCRL1 expression independently predicts poor prognosis in diffuse large B cell lymphoma (DLBCL), with high expression significantly reducing overall and progression-free survival in R-CHOP-treated patients 6. Furthermore, FCRL1 expression marks tumor-infiltrating immune cells associated with resistance to checkpoint inhibitor therapy 7. These findings establish FCRL1 as both a promising diagnostic/prognostic biomarker and therapeutic target in B cell lymphoproliferative disorders.