The fact that both, the Molteni study using 5-HTT deficient rats and our study using 5-HTT deficient mice revealed 5-HTT genotype dependent alterations in baseline Arc expression may be explained by a compensatory mechanism in response to the lifelong altered extracellular and intracellular 5-HT-levels, which could be the neurobiological correlate of heightened anxiety. This evidence concerns the gene ARC and Anxiety.