The study of primary immunodeficiency diseases has provided significant insights into how the immune system works. This notably includes in vivo immune responses to microorganisms, pathogens or opportunistic agents and in vivo control of reactivity to self, in relation to the identification of specific gene mutations. Hence, some of the key factors of the immune system have been unraveled. In this respect, studies in human conditions are essential because (i) they provide truly natural models of the immune system at work and (ii) because there are many known differences between the human and murine immune systems. Detailed investigations of newly described phenotypes and further delineation of known phenotypes which lack associated gene mutations represent as many new models for studying multiple facets of the human immune system. Appropriate in vitro and in vivo experimental models will be designed in order to gain a detailed understanding of newly identified molecules. It is also expected that the elucidation of immune system pathways will have an impact on therapeutic research. This proposal has been generated by a group of scientists with expertise in both fundamental and clinical research and aims at tackling some of these multifaceted questions. These studies will benefit from the well-characterized cohorts of patients with specific primary immunodeficiencies (PIDs) that have been constituted over the years. The project has 3 major subheadings: 1) Primary immunodeficiencies (PIDs) of DNA repair pathways, 2) PIDs of lymphocyte cytotoxicity/cell death pathways; 3) Therapeutics for primary immunodeficiencies. Scientists who take part in the project will address these various topics as a function of their respective fields of expertise. Also, as observed in the past, the unexpected observation of new phenotypes might significantly impact on research orientations, as a function of their potential interest and feasibility of their analysis.