Unfortunately, 5 children (4 in Paris, 1 in London) at 24-68 months from gene therapy developed an acute lymphoblastic leukemia (ALL) mainly because complex mutagenesis started with the insertion at 11p13 of a single copy of the defective retrovirus, containing the normal IL2RG gene but also its enhancer, near the promoter of LMO2 (LIM domain only 2), known oncogene of T lymphocytes then aberrantly transcribed and expressed (insertional mutagenesis) [121,122]. The gene discussed is LMO2; the disease is acute lymphoblastic leukemia.