Insertional oncogenesis observed with ex vivo retrovirally mediated gene transfer, using Moloney murine leukemia virus, of the interleukin-2 receptor γ-chain gene into CD34+ cells in patients with severe combined immunodeficiency. Insertional mutagenesis involving LMO2 and identified the potential involvement of other proto-oncogenes, including CCND2 and BMI1. Genomic integration bias in and near gene coding regions and around transcription start sites. This evidence concerns the gene BMI1 and immunodeficiency disease.