['Gold nanoparticles have recently been investigated with respect to biocompatibility according to their interactions with cells. The purpose of this study was to examine cytotoxicity and apoptosis induction by well-characterized gold nanoparticles in human breast epithelial MCF-7 cells.', 'Apoptosis was assessed by TUNEL, cytotoxicity by MTT assay and caspase 3, 9, p53, Bax and Bcl expression by real-time PCR assays.', 'Gold nanoparticles at up to 200 μg/mL for 24 hours exerted concentration-dependent cytotoxicity and significant upregulation of mRNA expression of p53, bax, caspase-3 and caspase-9, whereas expression of anti- apoptotic bcl-2 was down-regulated.'] Do gold nanoparticles induce apoptosis in MCF-7 human breast cancer cells?