Abnormal c-Fos expression in TetTag mice containing fos-EGFP.

Molecular and genetic techniques now allow selective tagging and manipulation of the population of neurons, often referred to as "engram cells," that were active during a specific experience. One common approach to labeling these cells is to use the transgenic mouse (TetTag). In addition to tagging cells active during learning, it is common to examine the reactivation of these cells using immediate early gene (IEG) expression as an index of neural activity.