CD34 and severe combined immunodeficiency: D.B. Kohn et al. conducted a study on patients diagnosed with clinically severe combined immunodeficiency (ADA-SCID) caused by adenosine deaminase (ADA) deficiency, wherein they employed a gene therapy approach involving ex vivo transduction of autologous CD34+ hematopoietic stem and progenitor cells (HSPCs) with a self-inactivated lentiviral vector encoding human ADA.