MMRN1 encodes multimerin-1, a large homopolymeric protein with critical roles in hemostasis and cellular adhesion. The protein functions as a carrier for platelet factor V/Va, facilitating storage and stabilization within platelets and regulating thrombin generation upon release 1. MMRN1 serves as a ligand for integrins α-IIb/β-3 and α-V/β-3 on activated platelets and functions in the extracellular matrix to support platelet adhesive functions 1. The protein binds to von Willebrand factor through its A1A2A3 domains under high shear conditions, supporting platelet adhesion at arterial shear rates 1. Beyond hemostasis, MMRN1 marks highly primitive hematopoietic stem cells with enhanced quiescence and regenerative capacity, and its deficiency impairs stem cell self-renewal 2. In pathological contexts, MMRN1 is upregulated in acute myeloid leukemia and marks quiescent leukemic stem cells responsible for disease initiation 2. The protein also promotes renal cell carcinoma progression through AMPK/MMPs pathway activation 3. MMRN1 undergoes O-fucosylation on its EMI domain by POFUT3/POFUT4, which regulates protein secretion through endoplasmic reticulum quality control 4.