Human and mouse cells in culture are fused by treatment with a virus or chemical agent, yielding a hybrid cell containing both human and mouse chromosomes. (In this example only three human and two mouse chromosomes are shown.) Human chromosomes are unstable in such hybrids and are gradually lost during the outgrowth of individual clones of the hybrid cells. Therefore, a panel of hybrid cell lines is obtained, with each line containing a different complement of human chromosomes. Hybridization of a cloned gene to DNAs extracted from such a panel of hybrid cell lines can be used to map the gene to a specific human chromosome. In this illustration, a human probe hybridizes to DNAs of cell lines A and D, but not B and C. Since only hybrid cell lines A and D have retained chromosome 1, these results map the cloned gene to this human chromosome.