A large international study of patients with DNA ligase 4 deficiency, cernunnos-XLF deficiency and NBS who underwent HSCT demonstrated significantly reduced transplant-related mortality and improved survival in those patients receiving a modified cytoreductive chemotherapy conditioning regimen with reduced doses of chemotherapy agents, as compared to those receiving standard doses [127], confirming reports from smaller series of patients with NBS [13,128]. This evidence concerns the gene LIG4 and Cernunnos-XLF deficiency.