Differential inhibitory potencies and mechanisms of the type I ribosome inactivating protein marmorin on estrogen receptor (ER)-positive and ER-negative breast cancer cells

► Marmorin inhibited the proliferation of breast cancer cells in vitro and in vivo. ► MCF7 and MDA-MB-231 cells showed differentiated sensitivity to marmorin. ► Marmorin down-regulates estrogen receptor α expression in MCF7 cells. ► MCF7 and MDA-MB-231 cells shared discrepancy of apoptotic pathways caused by marmorin.