PARP9 and inflammatory bowel disease: In IBD, changes in IDO1 expression significantly correlated with the modulation of NAMPT (slope = 0.54; R2 = 0.69) and NNMT (slope = 0.51; R2 = 0.66) as well as PARP9 (slope = 0.34; R2 = 0.72), CD38 (slope = 0.44; R2 = 0.69), and several other NAD-consuming enzymes.