DNA unique or low-copy fragments were isolated from a genomic DNA library specific for the short (p) arm of human chromosome 5. These chromosome 5p-specific DNA fragments were used to analyze, by Southern blot experiments, somatic cell hybrids that retained either a normal chromosome 5 homolog or a homolog with a partial deletion of 5p, which was derived from either of two persons with the common human deletion syndrome, cri du chat or 5p- syndrome. In these studies, two classes of DNA fragments were identified, those located outside the region deleted in the persons with cri du chat and those located within the deleted region. This latter class of DNA probes will help to define, at the molecular level, a region of 5p that is critical in producing the phenotype associated with the cri du chat syndrome.