Okano T, et al. - In this retrospective review, researchers reviewed the medical records of cohorts undergoing hematopoietic stem cell transplantation (HSCT) for activated phosphatidylinositol-3-OH kinase-delta (PI3Kδ) syndrome type 1 (APDS1) at collaborating facilities in order to determine the clinical manifestations, laboratory findings, prognosis, and treatment of APDS1 and explore appropriate indications and methods of HSCT. Clinical manifestations of patients with APDS1 were found to show variance. HSCT was commonly indicated in the presence of life-threatening progressive combined immunodeficiency and massive lymphoproliferation. Relief in clinical symptoms was attained with fludarabine-based reduced intensity conditioning (RIC)-HSCT, but transplant-related complications were frequently reported, including graft failure.