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Cyclobutane dipyrimidines and less than mean value of 6-4 dipyrimidines are the two major classes of mutagenic DNA photoproducts produced by UV irradiation of cells. We developed a method to map cyclobutane dipyrimidines at the DNA sequence level in mammalian cells. The frequency of this class of photoproducts was determined at every dipyrimidine along the human phosphoglycerate kinase-1 (PGK1) promoter sequence and was compared to the UV-induced frequency distribution of mean value of 6-4 dipyrimidines. After irradiation of living cells containing active or inactive PGK1 genes, enzymatic or chemical cleavage at UV photoproducts, and amplification by ligation-mediated polymerase chain reaction, photofootprints were seen in all regions which bind transcription factors and appear as DNase I footprints. Photoproduct frequency within transcription factor binding sites was suppressed or enhanced relative to inactive genes or naked DNA with enhancements of up to 30-fold. Since photoproducts are mutagenic, this indicates that photoproduct (mutation?) hot spots may be tissue specific in mammals.