Arctiin is a lignan found in many plants of the Asteraceae family, particularly the greater burdock (Arctium lappa) and Centaurea imperialis, and in Trachelospermum asiaticum, Saussurea heteromalla., and Forsythia viridissima. It is the glucoside of arctigenin. Arctiin is a major lignan constituent of Arctium lappa and has anti-cancer properties in animal models. It was recently reported that arctiin induces growth inhibition in human prostate cancer PC-3 cells. However, the growth inhibitory mechanism of arctiin remains unknown. Arctiin induces growth inhibition and dephosphorylates the tumor-suppressor retinoblastoma protein in human immortalized keratinocyte HaCaT cells. Arctiin down-regulates cyclin D1 protein expression and that this at least partially contributes to the anti-proliferative effect of arctiin.