XIAP and immune system disorder: XIAP (X‐linked IAP) is indispensable for NOD2 signaling and familial mutations in XIAP that impact on its function cause severe immunodeficiency with variable clinical presentation, including early‐onset chronic colitis in ~20% of afflicted individuals (Bauler et al, 2008; Krieg et al, 2009; Damgaard et al, 2012, 2013; Speckmann et al, 2013; Pedersen et al, 2014).