Morusin exhibits a dose- and time-dependent inhibitory effect on murine and human breast cancer cells. IC50 is 9.48 μg/mL for normal mammary epithelial cells (MCF-10A); 2.03 and 1.87 μg/mL for murine breast cancer cells (4 T1 and EMT6); and 2.71 and 3.86 μg/mL for human breast cancer cells (MCF-7 and MDA-MB-231), respectively, the maximal inhibition of cell growth (>80 %) is obtained at 8 μg/mL. The apoptotic cells in morusin treated breast cancer cells are increased significantly in a dose-dependent manner. Morusin significantly inhibits the growth and clonogenicity of human colorectal cancer HT-29 cells. Morusin also inhibits the phosphorylation of IKK-α, IKK-βand IκB-β, increases expression of IκB-α, and suppresses nuclear translocation of NF-κB and its DNA binding activity. Dephosphorylation of NF-κB upstream regulators PI3K, Akt and PDK1 is also displayed. In addition, activation of caspase-8, change of mitochondrial membrane potential, release of cytochrome c and Smac/DIABLO, and activation of caspase-9 and -3 are observed at the early time point. Downregulation in the expression of Ku70 and XIAP is exhibited afterward. Morusin suppresses viability of prostate cancer cells, but little effect in normal human prostate epithelial cells. Morusin also reduces STAT3 activity by inhibiting its phosphorylation, nuclear accumulation, and DNA binding activity. In addition, morusin down-regulated expression of STAT3 target genes encoding Bcl-xL, Bcl-2, Survivin, c-Myc and Cyclin D1. It induces apoptosis in human prostate cancer cells by reducing STAT3 activity.