Unbalanced chromosome aberrations detected by routine chromosome diagnostic services can be used for gene mapping by gene dosage. This procedure, once discredited by early observations in Down’s syndrome, has now provided some of the most precise intrachromosomal gene localizations known and these are reviewed. Cytogeneticists have an obligation to see that every opportunity is taken to obtain mapping information from suitable cases. Interpretation of cytogenetic findings may be improved by application of appropriate gene dosage studies, and examples are described in which a diagnosis was made possible by such studies. Cytogeneticists may also contribute to human gene mapping by application of the technique of in situ molecular hybridization. The localization of the immunoglobulin kappa light chain genes (IGKV) to the short arm of chromosome 2 reported at this workshop provides the first example of the localization of an unassigned, unique DNA gene sequence by this method. Combined with the use of chromosomal translocations, resolution of assignments to chromosomal sub-bands is possible, and, as suitable DNA clones become available, the technique should have wide applicability in human gene mapping.