Gene therapy for rare inherited immune disorders has become a clinical reality. Especially for SCID, two major types of SCID (ADA-SCID, X-SCID) have been successfully treated by autologous stem cell based gene therapy. However, for the most common group of SCID, the SCID underlying recombination defects, this has not yet occurred due to the higher complexities of the affected genes involved. The aim of the current proposal is to fill the unmet medical need for the most common major category of SCID, recombination activating gene-1 (RAG-1) deficient SCID, by performing Stage I/II clinical trials using autologous hematopoietic stem cell based gene therapy. To this end we will develop novel safety assays, pre-GMP and GMP lentiviral batches and design and conduct multicenter, multinational clinical trials with input from regulatory authorities such as EMA and patient advocacy groups. The trial will be conducted with phenotypic, molecular (integration sites, therapeutic gene expression) and functional readouts and should lead to effective treatment for > 70% of all SCID patients in Europe. RECOMB forms the logical extension of highly successful previous EU consortia that have made the EU global leader in gene therapy for orphan immune diseases.