Among them, 18F-labelled (S)-1-(2-fluoroethyl)tryptophan ((S)-[18F]FETrp), a substrate of the IDO1 enzyme (Henrottin et al. 2015), and currently under clinical investigation (NCT05556473), has demonstrated very promising preclinical results in terms of IDO1 positive-tumour cell uptake (Henrottin et al. 2016; Xin and Cai 2017; Xin et al. 2019) and pharmacokinetic profile in (patient-derived) tumour xenografts and transgenic tumour mouse models (Xin and Cai 2017; Xin et al. 2019; Xin et al. 2020; Michelhaugh et al. 2017). The gene discussed is IDO1; the disease is neoplasm.