Recently emerging 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)." In contrast to conventional 
biomarkers, IVDMIA requires an algorithm to calculate the diagnostic score. The 
linear classifier is the preferred algorithm. When the number of diagnostic 
genes is n, each tumor is represented by a point in an n-dimensional space made 
from gene expression values. Diagnostic algorithms (linear classifier) make an 
(n-1)-dimensional plane in the n-dimensional space to separate two patient 
groups. Calculation of the diagnostic score is achieved by dimension reduction. 
Currently, IVDMIA is restricted to gene expression profiling, and will also be 
applied to malignancies other than breast cancer.