Mutations in recombination activating genes 1 and 2 (RAG1 and RAG2) cause a spectrum of severe immunodeficiencies ranging from classical T cell–B cell–severe combined immunodeficiency (T^–B^–SCID) and Omenn syndrome (OS) to an increasing number of peculiar cases. While it is well established from biochemical data that the specific genetic defect in either of the RAG genes is the first determinant of the clinical presentation, there is also increasing evidence that environmental factors play an important role and can lead to a different phenotypic expression of a given genotype. However, a better understanding of the mechanisms by which the molecular defect impinges on the cellular phenotype of OS is still lacking. Ongoing studies in knock‐in mice could better clarify this aspect. Hum Mutat 27(12), 1174–1184, 2006. © 2006 Wiley‐Liss, Inc.