Cytogenetic RFLP (Restriction Fragment Length Polymorphism) Maps Using Radiation Hybrid RH Mapping-Due to technical limitations, DNA sequences more than a few hundred kilobase pairs long can not be easily arranged in a linear order.

Therefore, high resolution maps can not be easily prepared. Radiation hybrid (RH) mapping is a new method using somatic cell genetics, which allows ordering of DNA markers having millions of base pairs of DNA, with a resolution of 500kb.

RH mapping with PFGE (pulse field gel electrophoresis) has actually been used to construct a high resolution map of a 20Mb (20 x 106 bp) DNA segment belonging to the long arm of human chromosome 21 (Cox et al., 1991).