While the improved odor discrimination and learning behavior showed only little variability, the significantly impaired memory performance observed in GluR-BΔFB mice was highly variable among individual animals compared with control littermates (Figure 2E). This variability in olfactory memory was reflected in the level and extent of Cre-recombinase expression in forebrain of transgenic TgCre4 mice, as visualized by Cre-activity in the Cre-indicator mouse line R26R. We observed that onset and extent of Cre-recombinase expression in different forebrain regions varied among individual TgCre4 mice (Figure 3A), which could also be directly visualized by immunohistochemistry with a Cre-antibody (unpublished data). As this variability persisted after several backcrosses, and Southern blot analysis revealed no differences of transgene integration or number among animals (Figure 3B), it could not be attributed to genetic differences but rather to epigenetic mechanisms.