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Introduction: Anti-MAG neuropathies are associated with an IgM monoclonal gammopathy of undetermined significance (MGUS) or with a malignant haemopathy. Our objective was to determine whether the presence of a haemopathy or somatic mutations of MYD88 and CXCR4 genes influences disease presentation and response to rituximab (RTX). Methods: We included 79 patients (mean age 74 years, disease duration 9.68 years) who had a bone marrow aspiration with morphologic and immunophenotypic analysis. MYD88L265P and CXCR4 mutations were analysed in peripheral B cells. Information collected included: inflammatory neuropathy cause and treatment sensory sum score (ISS), MRC testing, overall neuropathy limitation scale (ONLS), Rash-built Overall Disability Score (RODS), ataxia score, anti-MAG titres, peak IgM dosage, neurofilament light chain levels, motor and sensory amplitudes, motor unit index (MUNIX) and motor unit size index (MUSIX) sum scores. Efficacy of RTX was evaluated at 12 months in 26 patients. Results: Malignant haematological disorders were discovered in 17 patients (22%): 13 Waldenstrom macroglobulinemia, 3 marginal zone lymphoma and one mantle cell lymphoma. MYD88L265P mutation was detected in 29/60 (48%) patients and CXCR4 in 1 single patient. Disease severity, biological and electrophysiological data and response to RTX were comparable in patients with MGUS/lymphoma and patients with/without MYD88L265P mutation. ISS was lower and MUSIX higher in patients improved by RTX. Conclusions: MYD88L265P mutation and underlying haemopathies are not predictive of a more severe disease. However, in cases of resistant and progressive neuropathy, they provide an opportunity to prescribe newly available drugs such as Bruton tyrosine kinase inhibitors. [ABSTRACT FROM AUTHOR]