" But some mRNAs can take a long time to break down in a cell. This means that when a stem cell starts to differentiate, there will be a period when it still contains many of the stem cell mRNAs. Happily, when the stem cell starts differentiating, it switches on a new set of miRNAs. These target the residual stem cell mRNAs and accelerate their destruction. This rapid degradation of the pre-existing mRNAs ensures that the cell moves into a differentiated state as quickly and irreversibly as possible "

― Nessa Carey , The Epigenetics Revolution