Several known GusA substrates coincide with both endogenous and environmental factors implicated in CD: nicotine [33] from tobacco smoke, the only environmental factor consistently predisposing to CD [5], and hormones or drugs [27,28] recognized by host transporters (MRP1/MDR1) or receptors (AhR) that are crucial in IBD [29–32]. The gene discussed is AHR; the disease is Cowden disease.