Locus content maps are derived from monosomic or disomic chromosomes broken by radiation, shearing, or other clastogen, the fragments being distributed among clones by dilution or incorporation into the cells of another species and scored for segregation of markers. Locus content maps provide evidence about radiosensitivity of chromosome regions, support for order, and approximate location. Omission of the most aberrant and least informative clones increases efficiency of localization. Correct analysis must allow for preferential retention of certain sequences, monosomy or polysomy of donor chromosomes, and error filtration. Combination of these refinements extracts substantially more information from fewer clones. Because of unmodeled peculiarities in the data, the best analysis does not recover the physical map but roughly localizes markers that may be monomorphic and therefore unsuitable for linkage mapping. As with linkage for polymorphic loci, distance in the composite map should be confirmed by physical methods.