ERBB4 and Anxiety: In addition, we did not observe significant differences in the number of transitions (control: 6.0 ± 0.27; Erbb4cKO: 6.30 ± 0.26; p = 0.448) (Figure 6C) and ratio of time spent in the dark over the brightly illuminated area (control: 1.60 ± 0.21; Erbb4cKO: 1.77 ± 0.32; p = 0.660) in the dark-light box test (Figure 6D), indicating that Erbb4 deficiency in Pet1+ serotonergic circuits have no effect on anxiety levels.