Schematic structure of the intestine-specific promoters and operator. (A) Promoter/enhancer fusion constructs. Four intestine-specific promoters were cloned into a pGL3-Basic vector to drive luciferase expression. In addition, the human mucin-2 promoter (HMUC2) sequence was fused to the human sucrase–isomaltase (HSI) and rat intestinal trefoil factor (RITF) regions to drive luciferase expression. (B) Schematic structure of the synthetic 116 bp operator, which consists of the two important components: LacO1 and LacO2.