In the current study, we treated murine 4T1 breast cancer cells with the highly effective PTEN inhibitor VO-OHpic, and showed that VO-OHpic effectively inhibited PTEN gene and protein expression levels, accompanied by significantly enhanced cell proliferation and increased colony-forming ability, altered the cell cycle distribution, and increased the proportion of S phase cells, suggesting that the rapid proliferation of 4T1 cells might be related to decreased expression or activity of PTEN in the cells. The gene discussed is PTEN; the disease is breast carcinoma.