Epigenetic clocks as quantitative readouts of epigenetic drift

Epigenetic drift leads to gradual alterations of the epigenome that have been harnessed (particularly at the DNA-methylation level) to build epigenetic clocks. However, the functional significance of the signals these clocks capture remains unclear. We discuss the evolution of the concept of epigenetic drift and examine how epigenetic clocks have emerged as quantitative readouts of this phenomenon.