Allogeneic hematopoietic stem cell transplantation (HSCT) has not been a therapeutic option in ataxia telangiectasia (AT) due to overwhelming toxicity of conditioning in the context of the global DNA repair deficiency. Furthermore HSCT is unable to cure neurological involvement of AT. We report on a Turkish child with a Hyper IgM phenotype disorder, in which clinical aspects of AT were absent and thus, AT not diagnosed. He was transplanted with a reduced toxicity, but full intensity conditioning regimen comprising treosulfan, fludarabine and ATG. The peritransplant period was uneventful and the patient was discharged at day +57. 8 months after HSCT, the patient developed hepatopathy with monoclonal gammopathy of unclear significance and died due to hepatic failure and encephalopathy at the age of 32 months. Post mortem high throughput sequencing revealed a mutation in the ATM gene.