Tagging neural ensembles by Arc catFISH. A, After neuronal activation, Arc mRNA is initially localized in the nucleus (left panel). Within 30 minutes, the mRNA translocates outside the nucleus (right panel). B. The Arc expression pattern in a neuronal ensemble is depicted in two different situations. In the upper panel, the subject was exposed to context A at two time points, firstly at 30 minutes and secondly 5 minutes prior to sacrifice and fixation (A/A). In the lower panel, the animal was first exposed to context A and then to a distinct context B (A/B). Exposure to the A/A condition led to induction of Arc localization both inside and outside of the nucleus in the same neurons, while A/B exposure induces Arc in mainly two distinct populations albeit with some overlap.