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Abundant expression of thymidine phosphorylase (TP) mRNA and protein was observed in human carcinoma tissues relative to the corresponding normal tissues. However, the distribution of cells that express TP has not yet been determined. In this study, we examined enzyme activity of TP in 24 primary colorectal carcinoma tissues and the corresponding normal tissue. Using in situ reverse-transcription polymerase chain reaction (RT-PCR), a new methodology to determine the localization of genes of interest objectively and clearly, cell distribution was compared. The expression of TP mRNA is observed in normal infiltrating cells adjacent to colon carcinoma cells by in situ RT-PCR. Similar to the protein activity, the average TP mRNA expression (amplified products/×400 field) in cases of Dukes C and D is 76.8, while that of Dukes A and B is 42.5, being significantly different (p = 0.001). Moreover, the number of amplified products in colon carcinoma tissues showed a significant correlation with the enzyme activity evaluated by high-performance liquid chromatography (p < 0.01, R = 0.76). In conclusion, the expression of TP mRNA is localized in stromal tissues as determined by in situ RT-PCR and can reflect the level of TP enzyme activity.Copyright © 2002 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]