Reversion mosaicism has subsequently been found in some PIDs such as X-linked severe combined immunodeficiency (X-SCID) (5, 7–13), recombination activating gene 1 (RAG1) deficiency (14, 15), CD3ζ deficiency (16–19), and Wiskott–Aldrich syndrome (WAS) (20–36). This evidence concerns the gene RAG1 and severe combined immunodeficiency.