Three structurally related 6-C-methyl flavonoids isolated from Pinus densata, including 5,4'-dihydroxy-3,7,8-trimethoxy-6-C-methylflavone (PD1), 5,7,4'-trihydroxy-3,8-dimethoxy-6-C-methylflavone (PD2), and 5,7,4'-trihydroxy-3-methoxy-6-C-methylflavone (PD3), were tested for their ability to inhibit