The retinoid N-(4-hydroxyphenyl)retinamide (4-HPR also known as fenretinide) is a potent inducer of apoptosis in breast cancer cells. We observed a 4.5-fold reduction in 4-HPR-mediated apoptosis in MCF-7 breast cancer cells transfected with HER2/neu (MCF-7/HER2) as compared with the parental MCF-7 (MCF-7/WT) cells. Blocking HER2/neu with trastuzumab (Herceptin TM ) led to a sixfold increase in 4-HPR-induced apoptosis in HER2/neuoverexpressing cells. These data indicate that HER2/neu reduces the sensitivity of breast cancer cells to 4-HPR. We showed previously that nitric oxide (NO) is essential for 4-HPR to induce apoptosis in breast cancer cells. The inhibitory effects of the 4-HPR and trastuzumab combination correlated with the amount of NO produced in HER2/neu-overexpressing cells. When a NO synthase (NOS) inhibitor was used to block NO production, decreased apoptosis by the 4-HPR and trastuzumab combination was observed. Furthermore, 4-HPRmediated NOSII expression was lower in MCF-7/HER2 than MCF-7/WT cells, but was increased by trastuzumab in HER2/neu-overexpressing cells. Here we report the novel findings that HER2/neu reduces the ability of 4-HPR to induce apoptosis in breast cancer cells, and that one mechanism by which HER2/neu increases the resistance of breast cancer cells to 4-HPR is by decreasing NOSII-mediated NO production.