miR-671-3p mimic accelerated the apoptosis of MDA-MB-231 cells. (A) Effect of miR-671-3p mimic on cell apoptosis in terms of the caspase-3/7 activity, as determined by an Apo-ONE Homogeneous Caspase-3/7 assay. (B) Effect of miR-671-3p mimic on cell apoptosis, determined by fluorescence microscopy following Hoechst 33342/PI double-fluorescent chromatin staining. (C) Viable and apoptotic cells were observed under a microscope following Hoechst 33342/PI staining (magnification, x200) at 96 h. Viable cells exhibited Hoechst 33342-positive and PI-negative staining; early apoptotic cells exhibited Hoechst 33342-positive and PI-negative staining, with blue fragmentation in the cells; late apoptotic cells exhibited Hoechst 33342-positive and PI-positive staining, with red fragmentation in the cells; and necrotic cells exhibited PI-positive staining with debris signals. *P<0.05 and **P<0.01 vs. negative control at 0 h. miR, microRNA; PI, propidium iodide.