Inclusion criteria :  Renal transplant patients with a plasma of BK-v ≥ 103 copies/ml confirmed on 2 consecutive blood BK-V PCR values for a period of ≥ 1 month Men or women aged at least 18 years old - No other organ transplant except kidney transplant - Informed patient who did not object to participating in the study - Beneficiary of a social security scheme or entitled  Exclusion criteria  Renal transplant patients with a plasma viral load of BK-v ≥ 103 copies/ml on an isolated BK-v PCR value Progressive cancer apart from localized skin cancers - Occurence of acute rejection during the month prior to inclusion - Chronic progressive infectious disease (tuberculosis, replicating viral hepatitis Bor C, HIV infection), - Patient who has not had a biopsy to confirm or not the diagnosis of BKv nephropathy. - Patient under guardianship / curatorship - Patient under State Medical Aid