A simple and accurate genomic primer extension method has been developed to detect ultraviolet footprinting patterns of regulatory protein-DNA interactions in mammalian genomic DNA. The technique can also detect footprinting or sequencing patterns introduced into genomic DNA by other methods. Purified genomic DNA, containing either damaged bases or strand breaks introduced by footprinting or sequencing reactions, is first cut with a convenient restriction enzyme to reduce its molecular weight. A highly radioactive single-stranded DNA primer that is complementary to a region of genomic DNA whose sequence or footprint one wishes to examine is then mixed with 50 micrograms of restriction enzyme-cut genomic DNA. The primer is approximately 100 bases long and contains 85 radioactive phosphates, each of specific activity 3000 Ci/mmol (1 Ci = 37 GBq). A simple and fast method for preparing such primers is described. Following brief heat denaturation at 100 degrees C, the solution of genomic DNA and primer is cooled to 74 degrees C and a second solution containing Taq polymerase (Thermus aquaticus DNA polymerase) and the four deoxynucleotide triphosphates is added to initiate primer extension of genomic DNA. Taq polymerase extends genomic hybridized primer until its polymerization reaction is terminated either by a damaged base or strand break in genomic DNA or by the addition of dideoxynucleotide triphosphates in the polymerization reaction. The concurrent primer hybridization-extension reaction is terminated after 5 hr and unhybridized primer is digested away by mung bean nuclease. Primer-extended genomic DNA is then denatured and electrophoresed on a polyacrylamide sequencing gel, and radioactive primer extension products are revealed by autoradiography. By using this method we demonstrate that it is possible to footprint with ultraviolet light, in intact monkey cells, regulatory protein--DNA interactions along a single copy of a simian virus 40 viral genome integrated into the monkey genome.