Loss of function of proteins involved in the IL-2RG signaling pathway leads to defective lymphoid development and severe combined immunodeficiency (XSCID). Due to the requirement for proliferative stimuli from IL-2RG receptor complexes, correction of the receptor subunit provides an opportunity for selective expansion of progenitor cells with the corrected phenotype. In human bone marrow cells, retroviral vectors have successfully transferred a normal copy of IL-2RG and restored function of signaling, lymphoid development and immune function. However, these attempts have recently revealed an unfortunate problem with oncogenic integration of the delivery retroviral vector. Therefore, alternative strategies for restoring proper expression of the receptor subunit will be necessary for curing patients with SCID. We propose to generate mouse models for SCID where the IL2RG locus contains small insertions, a restriction site for the yeast enzyme I-scel which creates a stop codon, or point mutations, which are found in human XSCID patients. These engineered mutations will then be corrected by either inducing double strand break (DSB) repair using I-scel and supplying a wild-type copy to be used as a template for homologous recombination, or by delivering an homologous wild-type DNA sequence using the single-stranded DNA particle, adeno-associated virus (AAV), In order for human XSCID to be corrected using one of these methods, a model system must be developed to examine the effect of the mutations on normal development, to identify the proper cell type to be targeted and to determine the efficiency of gene conversion and subsequent restoration of immune function. [unreadable] [unreadable]