Microscopic observation of H&E staining (Figure 7D) showed that the TDO2i group had no invasive carcinoma lesions (0 of 7) but more cases of mild-to-moderate dysplasia (early lesions, 5 of 7, 71%) on their tongues, whereas 6 cases of invasive carcinoma (6 of 8, 75%), 2 cases of preinvasive carcinoma (severe dysplasia or carcinoma in situ, 2 of 8, 25%), and no cases of mild-to-moderate dysplasia were noted in the untreated group (Figure 7E), indicating that administration of the TDO2 inhibitor significantly suppressed the formation of OSCC in 4NQO-treated mice (P < 0.01). Here, TDO2 is linked to carcinoma.