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An extra Y chromosome(s) is occasionally found in patients with various hematologic neoplasias; however, an association with hereditary blood diseases is unknown. In a child with β-thalassemia who received a transplant with sex-mismatched umbilical cord blood and neonatal blood, fluorescent in situ hybridization (FISH) with probes to X and Y chromosomes revealed an extra Y signal in 19.3% of the hepatocytes and in 38.9% of the pretransplant peripheral blood cells, yielding YY/Y ratios of 0.24 and 0.64, respectively. In granulocyte colony-stimulating factor-mobilized autologous peripheral blood stem cells, the FISH ratio of interphase XYY cells to XY cells was 1.64, and there were 3.4 times more G-banded XYY metaphases than normal metaphases. Both FISH and polymerase chain reaction persistently demonstrated posttransplant mixed chimerism. There was a greater proportion of residual autologous XYY cells than XY cells with a mean posttransplant YY/Y ratio of 1.56 (n = 13) (1 SD = 0.33; range, 1.10 to 2.17). In vitro clonogenic assays yielded a normal number of colony-forming units but no growth in cultures without growth factor supplement. This study suggests that hematopoietic stem cells with an extra Y chromosome may upregulate cell growth in response to cytokine stimulation. A posttransplant preponderance of XYY cells might be attributable to an extra Y chromosome in hematopoietic stem cells withstanding the myeloablative conditioning regimen.