Each patient with nephrotic syndrome should benefit of a kidney biopsy. For each patient we will conserved 2 dry tubes frozen at the time of biopsy. If we confirm the diagnosis of MN with anti-PLA2R1 Ab, we propose to the patient to be included in this study. He will have a visit every months for the first three months and every three months after, with collection of the following: blood pressure, weight, creatinine, albumin, blood electrolytes, proteinuria / creatinine in sample 1 assay Ac anti-PLA2R1 on 3 ELISA (human, rabbit and mouse) In case of persistent refractory nephrotic syndrome after 6 months, or the appearance of a 30% increase in serum creatinine, treatment with rituximab (2 × 1 g IV 15 days) will be proposed. Clinical and biological monitoring will be continued every 3 months for 18 months.