Certain genes, known as proto-oncogenes may mutate to become oncogenes, which promote the development of colorectal cancer. In other cases, tumor-suppressor genes may be inactivated by mutations. Such mutations are usually acquired over the course of a lifetime. In addition, epigenetic alterations can silence genes that protect against the development of malignant transformations. This involves changes to the structure of chromosomal proteins called nucleosomes, rather than changes to the DNA itself.