Scanning electron micrograph (SEM) of a human breast cancer tumor cell. Clumps of cancerous (malignant) cells form tumors which possess the ability to invade and destroy surrounding tissues and travel to distant parts of the body to seed secondary tumors. Malignant cells proliferate and grow in a chaotic manner, with defective cell division retained within each new generation of cells. Variations also occur in size and structure of the cancer cell from its original type. Breast cancer is the most common cause of cancer in women. Magnification unknown.