Cancer is a group of diseases that exhibit uncontrolled growth, invasion of adjacent tissues, and sometimes metastasis (the movement of cancer cells through the blood or lymph). In cancer cells, the regulatory mechanisms that control cell division and limit abnormal growth have been disrupted, usually by the accumulation of several mutations. Cancer is therefore essentially a genetic disease. Although some cancer-related mutations may be heritable, most cancers are sporadic, meaning they arise from new mutations that occur in the individual who has the disease. In this chapter we will examine the connection between cancer and genes.