Gene expression profiling allows us to look into a tumor cell to see how genetic information is communicated and affects cell behavior. Data gathered during the development of the 70-gene assay (MammaPrint) set the stage for future assay development. Recently, a 21-gene assay (Oncotype DX) was recommended by ASCO and NCCN as a standard of care for newly diagnosed, stage 1 or 2, node-negative, estrogen-receptor-positive breast cancer patients.