A genome map in which STSs are positioned relative to one another on the basis of the frequency with which they are separated by radiation-induced breaks. The frequency is assayed by analyzing a panel of human–hamster hybrid cell lines. These hybrids are produced by irradiating human cells, which damages the cells and fragments the DNA. The dying human cells are fused with thymidine kinase negative (TK−) live hamster cells. The fused cells are grown under conditions that select against hamster cells and favor the growth of hybrid cells that have taken up the human TK gene. In the RH maps, the unit of distance is centirays (cR), denoting a 1% chance of a break occurring between two loci.