Gene therapy for X-linked SCID (X-SCID) with LV or retroviral SIN vectors has been shown to be successful and to lack the xenotoxicity problems previously observed when using γ-retroviral vectors.38, 39, 40 For ADA-SCID, both retroviral vectors (currently marketed as approved therapy under the name Strimvelis) and LV vectors have shown excellent clinical results that are comparable to HSCT with matched donors.10 This evidence concerns the gene ADA and severe combined immunodeficiency.