The BK virus (BKV) belongs to the Polyomaviridae family. The primary infection, generally asymptomatic, occurs during childhood. The virus then persists in latent form in the body, mainly in the epithelial cells of the kidney and urinary tract. Cellular immunosuppression favors BKV replication. It is responsible for pathologies of the renal-urinary tract such as BKV-associated nephropathy (BKVAN) in kidney transplant recipients, hemorrhagic cystitis (HC) in hematopoietic stem cell (HSC) recipients or ureteral stenosis.

To date, there is no specific antiviral treatment against BKV. The management of patients is essentially symptomatic and requires a multidisciplinary approach. It is therefore necessary to identify early prognostic markers for the occurrence of CH and to develop new therapeutic strategies.

Inclusion criteria:

• Adult patient aged 18 to 19 years
• Minor patient (from birth to 17 years old)
• HSC allograft patient managed by the pediatric oncology-hematology service of the HUS, between 01/01/2015 and 31/12/2019

Exclusion Criteria:

• Opposition of the patient (or parental authority holders) to participate in the study
• Biological resources from a patient who does not meet all inclusion criteria
• Patient deprived of liberty or under court protection
• Patient under guardianship or curatorship

HSC allograft patient managed by the pediatric oncology-hematology service of the HUS, between 01/01/2015 and 31/12/2019