Furthermore, in colorectal cancer, intestinal enterochromaffin (EC) cells and certain immune cells convert a portion of tryptophan (Trp) to 5-hydroxytryptamine (5-HT) via tryptophan hydroxylase 1 (TPH1), collectively exacerbating Trp starvation in the tumor microenvironment. Here, TPH1 is linked to colorectal cancer.