In food antigen-induced mast cell hyperplasia in mice, c-Kit+/FcE-RI+ mast cells in colon sections stained highly positive for hPGDS, and tetranor PGD metabolite, a mast cell-derived PGD2 metabolite, proved to be a specific biomarker for food allergy in patients [41,42]. The gene discussed is PTGDS; the disease is food allergy.