Lignin, the most abundant renewable aromatic polymer, has been shown to suppress the growth of mammalian tumor cells. Despite extensive studies on lignin structure and its engineering, there is little information on the biological activity of lignin in relation to its molecular structure or the molecular mechanisms by which lignin suppresses tumor cells in mammalian species. Here, we prepared microwave-assisted acid-catalyzed solvolysis lignin (MASL) from Japanese cedar and