NPC1 and inflammatory bowel disease: Although this seems low, it should be seen in perspective that the penetrance of IBD in patients with one of the three most common NOD2 mutations (homozygous and compound heterozygous) is only about 1.5%.51 Indeed, as patients with NPC1 have a shorter life expectancy,1 the true impact of NPC1 deficiency for the development of IBD may have even been underestimated.