A tumour marker is a substance that increases in concentration in the blood if a malignant tumour is present. The tumour marker is particularly used to monitor and check on the progression of a tumour and its treatment, which should reduce the tumour marker concentration. A high concentration of a marker is not sufficient to diagnose the corresponding tumour, although it will stimulate the doctor to investigate. The marker may be a substance secreted normally by healthy cells or only if they have become malignant. See PSA. Note that many forms of cancers do not have a tumour marker.