To test whether 5-HT is produced in the lung during the infection, the expression of genes encoding the enzymes involved in the 5-HT synthesis and degradation (Fig. 3a) as well as the 5-HT levels were assessed in total lung cells and lung homogenates, respectively, from Tph1+/+ and Tph1−/− littermates at different time points after the infection. The gene discussed is TPH1; the disease is infection.