We have used two different strategies to construct hybrid cells in which specific, individual human chromosomes or fragments thereof are maintained by direct selective pressure. Our first approach was to introduce a drug-resistance gene into human chromosomes using a retroviral vector, and to transfer the marked chromosomes via microcells into mouse cells. The second method was to fuse gamma-irradiated human cells with rodent cells to produce hybrids containing fragments of the human X chromosome. Such hybrid cell lines should greatly facilitate both human gene mapping and the isolation of human genes by molecular cloning. The gene-transfer technologies described here can also be used to construct cell lines in which the expression of genes involved in human diseases can be studied in vitro.