In 2006, invasive breast cancer will attack approximately 190,000 US women, and this malignancy will take the lives of approximately 41,000 patients. Breast cancer is the leading cause of cancer in US women (excluding skin cancers). Nanotechnology can potentially revolutionize the area of cancer research due to the small size and unique physical properties of nanoparticles, nanotubes, and nanowires. Keeping this in mind, this proposal presents a revolutionary approach of integrating molecular imaging and treatment in one step using gold nanoparticles. Gold nanoparticles have a unique property in that they change color from red to blue when they aggregate. By coating anti-oncogene-specific antibodies on the nanoparticles and targeting them to BT474 and MCF7 breast cancer cells, one can image the molecular receptor overexpressions in cancer cells. When the anti-oncogene-specific antibodies dock to their corresponding receptors in cancer cells, they will make nanoparticles aggregate and gradually change the color of the cell. The change in color will indicate cancer, and the intensity will indicate the level of progression of cancer. Nanoparticles with non-specific antibodies will not dock to the receptor on cancer cells and therefore will not produce color change. Once imaged and the cancer determined, focusing near infra-red light pulses to the nanoparticles will only heat the cancer cells and kill them selectively due to the unique optical-to-thermal transitions in these nanoparticles. The success of this technology can transform imaging and treatment of cancer to a single step for clinical applications.