The present invention relates to systems and methods for recording and assaying cellular events, in particular gene expression. The invention provides hereto a method of determining a cellular event of interest in a cell comprising providing a cell comprising a CRISPR-Cas system, wherein the CRISPR-Cas system comprises a guide RNA that targets a selected DNA sequence and a Cas protein capable of modifying the selected DNA sequence; whereby a nucleic acid molecule encoding at least one of the guide RNA or Cas protein is operably connected in the cell with a regulatory element comprising a promoter responsive to the cellular event, and whereby expression of at least one CRISPR-Cas system component is driven by the promoter; and determining cellular event of interest based on detection of the modification of the selected DNA sequence.