To investigate if serum FLCs can be considered a biomarker of both disease activity and effectiveness of RTX treatment, we retrospectively analyzed the serological profile in 4 (#10–13) out of our 13 MuSK-MG patients, before and 3–8 weeks after RTX infusion, when CD19+ cell count was decreased by more than 90%. This evidence concerns the gene MUSK and myasthenia gravis.