A common in vitro response for many chemopreventive and antitumor agents, including some cyclooxygenase inhibitors, is the increased expression of nonsteroidal anti-inflammatory drug-activated gene (NAG)-1/macrophage inhibitory cytokine (MIC)-1/prostate-derived factor (PDF). The experimental anticancer drug 2-(4-amino-3-methylphenyl)-5-fluorobenzothiazole (5F203) was a potent inducer of NAG-1 expression, and in MCF-7 cells, it inhibited cell growth and induced apoptosis. NAG-1 small interfering RNA blocked NAG-1 expression and 5F203-induced apoptosis in MCF-7 cells, indicating that NAG-1 may mediate the apoptosis and anticancer activity. One mechanism by which 5F203 increases NAG-1 expression is by increasing the stability of NAG-1 mRNA, dependent of de novo protein synthesis. Extracellular signal-regulated kinase (ERK) 1/2 phosphorylation was increased by 5F203, and inhibition of ERK1/2 phosphorylation abolished the induction of NAG-1 protein expression and increased the stability of NAG-1 mRNA. Thus, 5F203 regulates NAG-1 expression by a unique mechanism compared with other drugs. A mouse orthotopic mammary tumor model was used to determine whether 5F203 increased NAG-1 expression in vivo and suppressed tumor growth. Treatment of the mice with Phortress, the prodrug of 5F203, increased the in vivo expression of NAG-1 as measured by real-time reverse transcription-polymerase chain reaction from RNA obtained by needle biopsy, and the expression correlated with a reduction of tumor volume. These results confirm that NAG-1 suppresses tumor growth, and its in vivo expression can be controlled by treating mice with anticancer drugs, such as Phortress. Drugs that target NAG-1 could lead to a unique strategy for the development of chemotherapeutic and chemopreventive agents.