The creation of high-resolution maps is central to genetics: maps allow genes to be associated with phenotypes and can be used as tools for gene cloning. Maps for the human genome and other mammalian species are being created both by classical recombination methods and by physical mapping techniques. One powerful method for creating physical maps is the use of irradiation and fusion gene transfer (IFGT) to make radiation hybrid maps. Goss and Harris who showed that chromosome fragments, generated by lethal irradiation of donor human cells, could be 'rescued' by fusion to rodent recipient cells originally developed this technology. Each resulting hybrid cell, which includes both hamster and human DNA, can be grown up to yield a hybrid cells or hybrid cell line of cells containing the same random subset of the human genome. A radiation hybrid panel (RH) consists of a number of different hybrid cell lines (sometimes just called hybrids or clones). The recombinant clone is tested for the presence or absence of each DNA marker (for example microsatellites). By estimating the frequency of breakage, and thus the distance between markers, it is possible to determine their order along chromosome. The successful use of RH panels to map the human genome has led to the development of WG-RH (whole genome radiation hybrid) maps in other species, for example of the porcine genome. This map provides a resource for rapid, large scale physical mapping of the swine genome and facilitates resolving genetic and physical distances prior to designing strategies for positional candidate cloning of the gene(s) contributing to economically important traits. WG-RH panels have now been used to create chromosomal maps in several species: rat, horse, bovine, cat, dog and mouse.