We previously isolated an interfering transbody, 4MH2, which penetrated the cytosol of living cells and preferentially hydrolyzed the target Her2 (ErbB2) mRNA, resulting in Her2 gene silencing followed by apoptotic cell death in Her2-overexpressing breast cancer cells. Here, we report the apoptotic cell death mechanism mediated by 4MH2-induced Her2 gene silencing in Her2-overexpressing SK-BR-3 breast cancer cells, in comparison with a small interfering RNA (siRNA) targeting Her2 mRNA ($Her2_{18}$-siRNA). 4MH2 induced $G_0/G_1$ cell cycle arrest to cause apoptotic cell death in SK-BR-3 cells by triggering specific signaling pathways associated with Her2 knockdown, including upregulation of $G_0/G_1$ cell cycle arrest-associated $p21^{Cip1}$ and $p27^{Kip1}$, downregulation of cyclin D1, inhibition of Akt phosphorylation, and downregulation of antiapoptotic Bcl-xL, which are comparable to those mediated by $Her2_{18}$-siRNA. Our results suggest that 4MH2-mediated Her2 gene silencing can trigger the downstream signaling pathways caused by Her2 downregulation, comparable to those mediated by the corresponding siRNA.