Patient: Female, 32-year-old Final Diagnosis: Atypical hemolytic uremic syndrome Symptoms: Kidney injury • loss of the graft • microangiopathic hemolytic anemia • thrombocytopenia Medication: — Clinical Procedure: Dialysis • plasma exchange • plasmapharesis Specialty: Nephrology Objective: Rare disease Background: Atypical hemolytic uremic syndrome (aHUS) is an ultra-rare disorder characterized by over-activation and dys-regulation of the alternative complement pathway. The clinical presentation of the disease comprises thrombocytopenia, microangiopathic hemolytic anemia, and acute kidney injury. In most cases, aHUS is caused by genetic mutations in components of the alternative complement pathway. The risk of graft loss in patients after kidney transplantation with aHUS is dependent on the type of genetic mutation. Case Report: We present a case of a 32-year-old patient after a second kidney transplantation with atypical hemolytic uremic syndrome, whose clinical manifestation was triggered by the Shiga toxin-producing E. coli infection. Genetic testing revealed a new mutation (p.I342T) in the gene encoding complement factor B (CFB). Since 2 causative variants for aHUS have been described in the same exon of CFB gene, it might be supposed that the p.I342T variant has similar deleterious effects on CFB function. Despite the implemented treatment, graft function deteriorated and the patient had to return to a hemodialysis program and is currently on a waiting list for a third kidney transplant. The presence of the gene variant with increased susceptibility for aHUS and its recurrence after kidney transplantation makes the patient a good candidate for therapy with complement factor C5 inhibitors during and after the planned kidney transplantation. Conclusions: Our case confirms the importance of genetic testing in patients with any sign of thrombotic microangiopathy. The finding of Shiga toxin-induced HUS with typical clinical course should not limit our vigilance of complement-mediated HUS with high risk of renal failure.