X-linked severe combined immunodeficiency (SCID-X1) was the test-case for demonstrating the feasibility of using gene therapy targeting hematopoietic stem cells to reconstitute the human immune system and provide effective long-term treatment for a hereditary disease. A comprehensive and forward-looking review of the biological and medical research that helped advance this field is captured in the article “Gene Therapy for X-Linked Severe Combined Immunodeficiency: Where Do We Stand?,” published in Human Gene Therapy.