A large portion of hepatocytes are polyploid cells, thought to arise through endoduplication followed by aborted cytokinesis. However, several recent reports describing liver cell fusion with exogenously derived bone marrow cells have been published. The exact significance of this finding is unclear, because the adopted protocols involve ablation regimens, damaged livers and artificial injections of adult cells. By creating chimeric mice bearing distinct reporter genes (LacZ and GFP), we show that in an unperturbed setting, hepatocytes carrying both markers can be detected via immunohistochemistry and polymerase chain reaction analysis. To further corroborate these findings with a direct visualization of the chromosome content at the single-cell level, we performed genotype analysis via fluorescence in situ hybridization on XY/XX chimeric mice with a Y chromosome–specific paint and an X chromosome–specific bacterial artificial chromosome clone probes. Conclusion: This technique confirmed the occurrence of cell fusion in adult mouse liver. (HEPATOLOGY 2008.)