DAO and inflammatory bowel disease: Interestingly, in patients affected by IBD, four single-nucleotide polymorphisms (SNPs) of DAO have been detected, C47T (Thr16Met), C995T (Ser332Phe), C4106G (His646Asp), and G-4586T, as well as three HNMT SNPs, A595G, C314T, and A939G, resulting in amino acid substitution, which decreased protein activity and stability, thereby leading to impaired histamine metabolism regulation in the inflamed intestine [94,95,96].