Normal cells grow, divide, mature, and die in response to a complex set of internal and external signals. A normal cell receives both stimulatory and inhibitory signals, and its growth and division are regulated by a delicate balance between these opposing forces. In a cancer cell, one or more of these signals has been disrupted, which causes the cell to proliferate at an abnormally high rate. As they lose their response to the normal controls, cancer cells gradually lose their regular shape and boundaries, eventually forming a distinct mass of abnormal cells—a tumor. If tumor cells remain localized, the tumor is said to be benign; if the cells invade other tissues, the tumor is said to be malignant. Cells that travel to other sites in the body, where they establish secondary tumors, have undergone metastasis.