The brain stores memories through a neuronal ensemble, termed engram cells. A unique system was established to transfer neuronal population activity into light with discrimination between engram and non-engram cells using fluorescence proteins. This system revealed that engram sub-ensembles represent distinct pieces of information, which are then orchestrated to constitute an entire memory. In addition, some sub-ensembles preferentially reappear during post-learning sleep, and these replayed sub-ensembles are more likely to be reactivated during retrieval.