Scientists have developed a CRISPR-Cas9 based genome editing method for universal correction of disease-causing mutations in the CTLA4 gene, which most commonly manifest as a Primary Immunodeficiency. Current treatment involves monthly IV injections or weekly subcutaneous injections of a recombinant CTLA4-Ig fusion protein abatacept. This invention includes one-time infusion of a CTLA4-corrected autologous T cell therapy. The corrected patient cells are generated by ex vivo electroporation of a specific gRNA:Cas9 ribonucleoprotien (RNP) complex and cognate homology-directed-repair template (HDRT) targeting a functional copy of the CTLA4 gene within an intronic region of the endogenous CTLA4 gene. This combination allows for (1) highly efficient knockin (up to 70% in patient cells), (2) cell-type and context specific regulation of CTLA4 expression under natural promoter and regulatory elements, and (3) preservation of endogenous CTLA4 expression in uncorrected cells.