Gene expression assays that are used in daily clinical practice for treating 
early breast cancer patients have been introduced in the clinic only recently. 
This review discusses the development of these arrays, summarizes the validation 
of those that are commercially available and indicates how the information 
provided by these assays can help in the care of patients. The review also 
provides an extensive overview of commercially available assays focusing on 
MammaPrint, the first and only assay for breast cancer management that has been 
cleared by the FDA.