Although nucleolar protein nucleostemin (NS) is essential for cell proliferation and early embryogenesisand expression has been observed in some types of human cancer and stem cells, the molecular mechanismsinvolved in mediation of cell proliferation and cell cycling remains largely elusive. The aim of the present studywas to evaluate NS as a potential target for gene therapy of human breast carcinoma by investigating NS geneexpression and its effects on SKBR-3 cell proliferation and apoptosis. NS mRNA and protein were both foundto be highly expressed in all detected cancer cell lines. The apoptotic rate of the pcDNA3.1-NS-Silencer group(12.1-15.4±3.8%) was significantly higher than those of pcDNA3.1-NS (7.2-12.0±1.7%) and non-transfectiongroups (4.1-6.5±1.8%, P<0.01). MTT assays showed the knockdown of NS expression reduced the proliferationrate of SKBR-3 cells significantly. Matrigel invasion and wound healing assays indicated that the number ofinvading cells was significantly decreased in the pcDNA3.1-NS-siRNA group (P<0.01), but there were no significantdifference between non-transfected and over-expression groups (P>0.05). Moreover, RNAi-mediated NS downregulationinduced SKBR-3 cell G1 phase arrest, inhibited cell proliferation, and promoted p53 pathway-mediatedcell apoptosis in SKBR-3 cells. NS might thus be an important regulator in the G2/M check point of cell cycle,blocking SKBR-3 cell progression through the G1/S phase. On the whole, these results suggest NS might be atumor suppressor and important therapeutic target in human cancers.