CD34 and Immunodeficiency: In summary, vascular niche‐mediated expansion of steady‐state BM CD34+ cells has the potential to increase the accessibility and improve the outcome of success of HSPC gene therapy for patient populations with varied hematopoietic disease indications, from frequent immunodeficiencies (infectious or genetic), hemoglobinopathies (sickle cell disease) and anemias (Fanconi anemia) to rare orphan diseases.