In their review article, Perez-Castro et al. [73] focused on two Trp transporters: SLC1A5 and SLC7A5, both of which are up-regulated in 13 cancer types and in 9/13 cancers in which TDO2 is also up-regulated, in 9/12 cancers expressing FAMID, but only in 4/10 cancers expressing IDO1. Here, TDO2 is linked to cancer.