When the activity of individual genes it is longer required, there are two main mechanisms responsible for the “switching off”, mainly DNA methylation and the Polycomb protein complex.
Sometimes, these mechanisms lose their efficiency and some of the genes that should be “switched off” remain active. This, in turn, could lead to uncontrolled cellular proliferation, and tumorigenesis. These mechanisms, present both in lower organisms as well as in mammals, have always been thought to be separated and independent.