Biomarkers can be used to aid the diagnosis of a disease (particularly early detection of occult tumours), to aid in the staging of disease, to predict outcome of disease, to act as a surrogate for clinical progression and to monitor responses to treatment. It is therefore important when referring to biomarkers to state exactly which of these indications is being sought. There is a real clinical need for biomarkers in all these stages, especially where patient stratification prior to treatment can considerably improve the outcome of clinical trials. Cancer, being a heterogeneous disease, is unlikely to respond to any one specific treatment and biomarkers that could tell us if the patient and their cancer was resistant to certain drugs would be very useful and, indeed, these are being sought. In an analogous way these are even more necessary with regards to vaccines, which would benefit greatly from patient stratification and early prediction of disease response .