Piceatannol, isolated from the roots of Rheum undulatum, prevented oxygen radical formation and inhibited lipid peroxidation; the effective concentrations were 25 muM for a radical scavenging assay and 2.1 muM for a microsome assay. Using cDNA microarray analysis to determine which genes were modulated by piceatannol, 33 genes were up-regulated while 157 genes were down-regulated in the human gastric cancer cell line.