RB1 and neoplasm: These tumor-initiating actions linked to RB loss occur both in stem cells—in which normofunctioning RB keeps them in a quiescent state, their usual situation—and in postmitotic differentiated cells—in which RB mutation allows them to reintegrate into the cell cycle—and, especially in proliferative progenitor cells (called transitory amplifying cells in the oral epithelium), which constitute an intermediate step between stem cells and postmitotic differentiated cells.