With CRISPR-Cas1 and Cas2 and retrons you can keep track of the transcription history in a population of cells in a kind of log. Americans write this in Nature.

The biological cell is a very complex entity in which all kinds of events take place. One of the events in a cell is gene expression, in which DNA is read and converted into a protein or a functional RNA molecule. Gene expression is something that bioengineers would like to record, preferably while the cell is still alive. Santi Bhattarai-Kline, Seth Shipman and colleagues from Gladstone Institutes and UCSF developed a way to record the transcripts in a cell chronologically using a CRISPR array and a ‘retron’.