DNA double strand breaks occur constantly in every cell caused by metabolic products, radiation or chemicals. Several cellular repair mechanisms are available to detect and repair DNA breaks. Genetic defects of DNA repair proteins can lead to immunodeficiency, bone marrow failure and malignancies. The only treatment option for many of these diseases is the allogeneic hematopoietic stem cell transplantation (HSCT), although associated with increased toxicity to conditioning regimens. We study functional biomarker of classical DNA repair pathways in blood cells of patients with DNA repair defects by flow and mass cytometry (CyTOF) in order to identify disease-specific patterns.