Gene expression classifiers such as the 70-gene signature that reflect the biology of breast tumours have started to find their way into daily clinical practice. Several retrospective validation studies in breast cancer have established the prognostic value of the 70-gene signature (MammaPrint). The prospective observational RASTER study shows excellent 5-year distant recurrence-free intervals in 98.4% of patients who had a high clinical risk but who according to the 70-gene signature had a low risk. Particularly in patients aged 45 years or older with an oestrogen receptor (ER)-positive, HER2-negative tumour, diameter 1-2 cm, grade 2 there is prospective evidence that the 70-gene signature can make a useful contribution towards decision-making on adjuvant chemotherapy.