Several single nucleotide variations (SNVs) in the multidrug resistance-1 (MDR1) gene, the pregnane X receptor (PXR, also NR1I2) gene, and to a lesser extent, the farnesoid X receptor (FXR, also NR1H4) gene have been evaluated, and some linked to IBD susceptibility with varying degrees of success5–13; however, none have emerged as a clinically meaningful marker of IBD presence, drug response or disease severity. This evidence concerns the gene ABCB1 and inflammatory bowel disease.