In spite of the importance of phospholipase D (PLD) in cell proliferation and tumorigenesis, little is known about the molecules regulating PLD expression. Thus, identification of small molecules inhibiting PLD expression would be an important advance for PLDmediated physiology. We examined one such here, denoted "Triptolide", which was identified in a chemical screen for inhibitors of PLD expression using cell assay system based on measurement of PLD promoter activity.