There are two types of genes that can cause tumors, proto-oncogenes or tumor-suppressor genes. Tumors occur when a gene sequence gets inappropriately turned on or off respectively, thereby causing uncontrolled growth. This can occur by a number of mechanisms: damage from environmental factors such as radiation or chemicals, an error in the replication of DNA when the cell divides, even changes in diet or physical conditioning (obesity has been linked directly to several cancers). While a tumor may start as benign (little to no chance of metastasis or invasive growth), additional changes to DNA may turn off genes that were keeping it controlled or turn on genes that promote uncontrolled growth. There are some protective factors, but many of these changes are cumulative and increase over time. The older we get, the higher the chance that the right combination of errors occur.