Proliferation
Rapid multiplication of cells. A tumour's proliferation rate, often measured by the marker Ki-67, tells you how fast it is growing and how it may respond to treatment.
Proliferation is the net result of cell division outpacing cell death; in a healthy tissue the two are balanced, and a tumour is a tissue where the balance has tipped. Pathologists estimate it by counting cells in mitosis or staining for Ki-67, a protein present only in dividing cells; a high index means an aggressive tumour that may respond well to chemotherapy but relapse quickly, a low index a slower tumour that may suit hormone therapy or watchful waiting. 'Sustaining proliferative signalling' is the first hallmark of cancer, and 'anti-proliferative' describes any drug that slows division rather than killing cells outright.