IDO1 and hepatocellular carcinoma: In the HCC xenograft mouse model, Abrine, an IDO1 inhibitor, can down‐regulate the accumulation of the metabolite KYN through the Janus kinase 1–signal transducer and activator of transcription 1 (JAK1–STAT1) signalling pathway and inhibit tumour growth by increasing the infiltration of CD8+ T cells and decreasing Tregs.164