The term cancer is generally used for malignant neoplasms. A malignant tumour displays invasive growth, i.e. growth beyond its anatomical boundaries, and can result in the formation of distant metastases. Cancer arises from unregulated cell growth, due to alterations in the genome of cells. The cells consequently no longer react to regulatory signals. The accumulation of various genetic defects in a cell ultimately lead to cancer. Cancer is often preceded by a histologically recognizable premalignant stage, called dysplasia. During the past decades, considerable advances have been achieved in characterizing the genetic changes in cells which lead to the beginning of the cell growth. The most important genes involved in these processes are oncogenes, tumour suppressor genes and the so-called DNA mismatch repair genes.