SMAD3 and inflammatory bowel disease: The promoter regions of human and mouse Pdcdl1 (PDL1) and Pdcdl2 (PDL2) genes contain multiple putative binding sites for Smad3, a transcription factor that is activated by transforming growth factor (TGF)-β1 (54), the activity of which is reduced in IBD mucosa due to high levels of Smad7, an intracellular inhibitor of TGF-β1/Smad3 signaling (55, 56).