Several infants treated with autologous ex vivo gene therapy for newly diagnosed X-linked severe combined immunodeficiency (X-SCID) in a phase I/II trial have developed fully functioning immune systems, including T cells, B cells and natural killer cells. The results, which were published in the April 17, 2019, issue of The New England Journal of Medicine, mark the first time that first-line treatment with X-SCID gene therapy has resulted in an immune system that was able to generate not just T cells, but the major cell types of the innate and adaptive immune systems.