In western Europe today, a third of all people develop a malignant disease at least once in their lifetime. Treatment has improved, and for many diseases, such as leukaemia and testicular cancer, the prognosis is much better today than it was 20 years ago. Screening for early diagnosis has also led to lower mortality for diseases such as breast cancer and cervical cancer; many malignancies, however, are still diagnosed after the metastatic process has already started, indicating a poor prognosis.

Tumour markers are usually proteins associated with a malignancy and might be clinically usable in patients with cancer. A tumour marker can be detected in a solid tumour, in circulating tumour cells in peripheral blood, in lymph nodes, in bone marrow, or in other body fluids (ascites, urine, and stool). A tumour marker may be used to define a particular disease entity, in which case it may be used for diagnosis, staging, or population screening. Markers may also be used to detect the presence of occult metastatic disease, to monitor response to treatment, or to detect recurrent disease (table). Recently they have even been used as targets for therapeutic intervention in clinical trials.