Breast cancer cells. Confocal light micrograph of breast cancer cells. Nuclei are orange, and cytoskeletal (structural) proteins are green. The nucleus at lower right is dividing. Cancer cells divide rapidly in an uncontrolled manner. Clumps of cancerous cells form tumours, which can invade and destroy surrounding healthy tissue. Breast cancer is the most common form of cancer in women. It also occurs in men, although this is far rarer. If a tumour is detected early, it can be removed surgically. Often, however, the cancer has spread to other parts of the body by the time it has been discovered. In these cases, the prognosis is less good. Magnification unknown.