Conversely, TDO2 mRNA expression has been found in some tumour cell lines and also in human tumours, such as hepatocarcinoma, melanoma and glioblastoma (van Baren and Van den Eynde 2015), cutaneous melanoma (Pilotte et al. 2012) and primary uveal melanoma, in which mRNA expression is generally low but its increase is associated with poor prognostic markers (Terai et al. 2020). Here, TDO2 is linked to melanoma.