CREB1 and melanoma: The requirement of the methyl group at the 4′-position of the B-ring for melanogenesis in B16F10 melanoma cells was similar to that for the inhibition of the SIK2-mediated suppression of CREB activity in HEK293 cells, suggesting that 4′-O-methyl flavonoids may induce melanogenesis mainly due to the inhibition of SIK2.