SLC6A4 and depressive symptom measurement: This model benefits from a targeted disruption of the 5-HTT gene, that leads to increased amounts of depression- and anxiety-like behaviors in 5-HTT heterozygous (+/−) and homozygous (−/−) knockout, compared to wild type (+/+) mice (Bengel et al., 1998; Holmes et al., 2003; Lira et al., 2003; Zhao et al., 2006; Kalueff et al., 2007; Popa et al., 2008; Heiming et al., 2009; Lewejohann et al., 2010; Krakenberg et al., 2019).