Developments in breast cancer biology over the last year have brought molecular medicine closer to the clinic. Within the past year, two major advances have taken place. First, microarray-based expression profiling has shown promise with the preliminary demonstration that clustering techniques can predict clinical outcome in lymphoma, pediatric leukemia, and breast cancer. Data in breast cancer have demonstrated the ability of microarray-based expression profiling to detect tumor cells in peripheral blood samples, to predict chemotherapy responses in fine-needle aspiration samples in neoadjuvant chemotherapy, and, most importantly, to predict disease-free survival and overall survival from profiles in breast cancer surgical specimens. Second, in breast cancer genetics, CHEK2 was identified as one of what are likely to be many low-penetrance breast cancer susceptibility genes. These studies demonstrate the transition of basic biologic research to clinical application.