Perli, S.D. et al. Science http://dx.doi.org/10.1126/science.aag0511 (2016).

Tracking the magnitude or duration of biological activity in mammalian cells is technically challenging. Perli et al. devised a strategy that uses the CRISPR–Cas9 system to store such 'analog' information, called mammalian synthetic cellular recorders integrating biological events (mSCRIBE). In mSCRIBE, a DNA sequence necessary for Cas9 recognition is added downstream of a gene that encodes the small guide RNA, generating a self-targeting guide RNA (stgRNA) sequence. Then the expression of Cas9 is linked to a biological process of interest. When this process occurs, the Cas9 targets the stgRNA gene for cleavage and subsequent mutagenesis during DNA repair. These mutations serve as a recording of when the process occurred. The researchers show that mSCRIBE can be used to record induced acute inflammation events over time.