BDNF and inflammatory bowel disease: Gene-smoking interactions were further identified in other IBD risk genes (ATG16L1, IL23R, and CALM3), detoxification genes (GSTP1 and HMOX1), smoking-associated genes (CHRNA3, CHRNA5, PPP1R3C, and BDNF), and the inflammatory cytokine (IL1B) through a candidate gene approach.