Illustration of a cancer cell. A cancer cell is a cell in which certain genes, having undergone successive alterations, can no longer be repaired and in which the sequence that would normally allow apoptosis (programmed cell death) is altered. The cell will therefore duplicate with these alterations, develop more rapidly than healthy cells and pack together to form a cancer tumour. The membrane of the cancer cell and that of the nucleus are also irregular, unlike that of a healthy cell.