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