The adenoma-carcinoma sequence refers to a stepwise pattern of mutational activation of oncogenes (e.g. K-ras) and inactivation of tumour suppressor genes (e.g. p53) that results in cancer. An oncogene is a gene that has the potential to cause cancer. In tumour cells, these are often mutated or expressed at high levels. A tumour suppressor gene is a gene that is involved in dampening the cell cycle or promotion of apoptosis or both. Examples include inactivation of p53 (resulting in osteosarcoma, endometrial carcinoma, and many others) and DNA repair proteins (e.g. hereditary non-polyposis colorectal cancer, multiple endocrine neoplasia, BRCA).