The possibility that metaphase chromosomes can serve as a source of genetic material for making transgenic mice was suggested by our previous finding of the incorporation of human satellite DNA into mouse embryos that were injected with microdissected human centromeric fragments. In the present study, we further examined whether this chromosome transfer method can be used to generate transgenic mice containing a portion of human chromosome 4 spanning the Huntington's disease (HD) gene. For this purpose, we used an improved method of metaphase chromosome preparation that may minimize the potential for DNA damage. Using metaphase chromosomes prepared in this manner, chromosome fragments spanning the region of chromosome 4 containing the HD gene were microdissected, retrieved, and injected into fertilized mouse eggs. The injected eggs exhibited good viability and developed with a high efficiency when implanted into foster mothers. To determine whether the human DNA from the injected chromosome fragment had been incorporated into the mouse genome, embryos were harvested at 12.5 days of gestation (dg) and analyzed by in situ hybridization using a human Alu repetitive DNA probe. This analysis showed that most of the embryos contained cells with human Alu repeats. However, all of the embryos were mosaic, and the level of mosaicism was such that we were not able to determine the precise chromosomal origin of the human DNA insert. We discuss the possible basis for the mosaicism and the potential value of such mosaic animals for studying Huntington's disease.