Figure 1. A model of assembly and re-assembly of neurons that are activated during formation and retrieval of memory. Neurons are activated in response to stimuli that trigger memory formation and these activated neurons constitute a functional assembly, or an active neuronal network, in several brain regions. Re-activating this neuronal network is a critical process during memory retrieval. A large amount of evidence indicate that molecular and cellular traces of memory which were produced during memory encoding within the original neuronal network that was assembled in response to the initial memory-inducing stimuli persist in this network. An outstanding question is whether, and if so how, this network of neurons can be “re-assembled” at the cellular and synaptic levels when updating memory after retrieval. Addressing this problem is essential for not only in memory research but also in molecular systems neuroscience.