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cDNA microarray analysis of HeLa S3 cells treated with the G-quadruplex-interactive cationic porphyrin TMPyP4. Cells were treated with 100 µm TMPyP4 for 48 h, and mRNA was used to probe a 500-gene array. The expression of a number of oncogenes, including c-FOS, JUN-B, c-MYB, STAT-1, and c-MYC, was decreased specifically by this agent, and not by its positional isomer TMPyP2, which interacts with G-quadruplex DNA very poorly. The c-MYC oncogene is one of the most consistently down-regulated genes in this study and bears a sequence in its promoter that forms a G-quadruplex structure in vitro. For details, see 565 in this issue.