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Gammaretroviral vectors are an efficient means to effect gene therapy. However, genotoxicity from insertion at nonrandom sites can confer a competitive advantage to transduced cells, resulting in clonal proliferation or neoplasia. Six pig-tailed macaques (Macaca nemestrina) underwent total body irradiation and reconstitution with autologous stem cells genetically modified by a gammaretroviral vector overexpressing HOXB4. Two animals were euthanized owing to irradiation- or transplantation-associated toxicity, whereas the other 4 had successful reconstitution. Of the 4 macaques with successful reconstitution, 1 has no long-term follow-up information; 1 was euthanized owing to infection with simian varicella virus infection 18 months post-total body irradiation; and the 2 others are described herein as case Nos. 1 and 2. After being stable for 3 years, case No. 1 developed pancytopenia and petechiation, and after 2 years of stability case No. 2 developed anemia and thrombocytopenia. Despite therapy, the animals deteriorated and were euthanized. Gross findings included emaciation; case No. 1 also had hemorrhage, peritonitis, and cholecystitis. Histologically, bone marrow was hypercellular with predominately blast cells of all hematopoietic lineages, though with myeloid predominance, and with maturation arrest and blast cell dysplasia (myelodysplasia). Myelodysplasia was likely from a combination of insertional mutagenesis by the retroviral vector and overexpression of HOXB4. Consequences of myelodysplasia included the blood dyscrasias and, in case No. 1, hemorrhage, bacterial cholecystitis, hepatitis, and peritonitis.