We have developed an X-irradiation: cell fusion procedure that segregates segments of human chromosomes lacking selectable markers and have used this approach to construct somatic cell hybrids retaining fragments of human chromosome 4 as the only human material. To identify hybrids retaining a small chromosomal fragment in the region of the Huntington disease (HD) gene, we used Southern blot analysis to screen 72 hybrid lines for the presence or absence of seven chromosome 4 single-copy loci. These data, combined with in situ hybridization experiments, identified three hybrids of interest. One of these cell lines, C25, stably retains a 10,000- to 20,000-kb fragment of distal 4p in the vicinity of the HD gene, translocated to a hamster chromosome. Field-inversion gel electrophoresis revealed no evidence of rearrangements in the human DNA present in C25. In combination with similar radiation hybrids, C25 is a valuable tool for isolating DNA probes near the HD gene. © 1989.