In former studies using Tph2−/− mice conflicting results were obtained regarding anxiety-like behaviors,28, 29, 30 probably reflecting a confounding locomotion phenotype depending on the aversive nature of the context.19 Therefore, a series of behavioral paradigms were conducted to test different aspects of lifelong 5-HT deficiency on locomotor activity and anxiety-like behavior in different contextual settings. Here, TPH2 is linked to Anxiety.