Tehran University of Medical Sciences Iranian Journal of Allergy, Asthma and Immunology 1735-1502 24 6 2025 10 29 860 866 EN Anahita Razaghian Division of Allergy and Clinical Immunology, Department of Pediatrics, Hakim Children's Hospital, Tehran University of Medical Sciences, Tehran, Iran Zahra Alizadeh Immunology, Asthma and Allergy Research Institute, Tehran University of Medical Sciences, Tehran, Iran AND Pediatrics Center of Excellence, Children's Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran Isabelle Meyts Department of Pediatrics, University Hospitals Leuven, Leuven, BE AND Laboratory for Inborn Errors of Immunity, KU Leuven, BE Arash Kalantari Department of Pediatrics, Valiasr Hospital, Imam Khomeini Hospital Complex, Tehran University of Medical Sciences, Tehran, Iran Sahar Rostami Hir Immunology, Asthma and Allergy Research Institute, Tehran University of Medical Sciences, Tehran, Iran AND Pediatrics Center of Excellence, Children's Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran Marjon Wouters Department of Pediatrics, University Hospitals Leuven, Leuven, BE AND Laboratory for Inborn Errors of Immunity, KU Leuven, BE Mohammad Reza Fazlollahi Immunology, Asthma and Allergy Research Institute, Tehran University of Medical Sciences, Tehran, Iran AND Pediatrics Center of Excellence, Children's Medical Center Hospital, Tehran University of Medical Sciences, Tehran, Iran 2024 10 24 2025 04 27 Adenosine deaminase 2 (ADA2) deficiency is an autosomal recessive disease with varying degrees of clinical phenotypes and disease severity. The phenotypic spectrum of the disorder has expanded from vasculitis with stroke to include pure red cell aplasia, bone marrow failure, autoimmune cytopenia, lymphoproliferation, and variable degrees of immunodeficiency. Here, we describe two cases of ADA2 deficiency: one presented with an early-onset stroke that resembled an early-onset polyarteritis nodosa (PAN), and the other as an adult-onset vasculitis that progressed to severe neutropenia with recurrent infection and lymphoproliferation. Patient 1, a 10-year-old male, had a reported pathogenic ADA2 homozygote variant; c.139G˃C (p.Gly47Arg), and patient 2, a 34-year-old male, had a reported likely pathogenic homozygous ADA2 variant; c.578C>T (p.Pro193Lys). Our second patient was the first DADA2 patient who showed that DADA2 is not a static disease and can progress from vasculitis to bone marrow failure in the course of the disease. Therefore, the previous recommendation introducing anti–TNF-α as a preferred treatment for vasculitis manifestations and hematopoietic stem cell transplantation as the preferred treatment for bone marrow failure can no longer apply. We should consider HSCT for DADA2 patients from the very beginning. The Physician has to be aware of this monogenic disorder's varied presentation and multi-organ involvement. Early recognition and proper treatment are crucial for this potentially fatal disease. https://ijaai.tums.ac.ir/index.php/ijaai/article/view/4229 https://ijaai.tums.ac.ir/index.php/ijaai/article/download/4229/2252