diagnostic tools such as MammaPrint and oncotype-DX are beginning to have impact on clinical practice of breast cancer. They are based on gene expression profiling, i.e., gene expression analysis of a large number of genes. Their unique characteristic is the use of a score calculated from expression values of a number of genes, for which the Food and Drug Administration (FDA) created a new diagnostic category entitled "in vitro diagnostic multivariate index assay (IVDMIA)."