Severe combined immunodeficiencies (SCIDs) constitute a heterogeneous group of life-threatening genetic disorders that typically present in the first year of life. They are defined by the absence of autologous T cells and the presence of an intrinsic or extrinsic defect in the B-cell compartment. In three newborns presenting with frequent infections and profound leukopenia, we identified a private, heterozygous mutation in the RAC2 gene (p.G12R). This mutation was de novo in the index case, who had been cured by hematopoietic stem cell transplantation but had transmitted the mutation to her sick daughter. Biochemical assays showed that the mutation was associated with a gain of function. The results of in vitro differentiation assays showed that RAC2 is essential for the survival and differentiation of hematopoietic stem/progenitor cells. Therefore, screening for RAC2 gain-of-function mutations should be considered in patients with a SCID phenotype and who lack a molecular diagnosis.