Inflammatory bowel disease in infants and young children has been increasingly recognized as being associated with monogenic defects, particularly PIDs such as IL-10 and IL-10R deficiencies, chronic granulomatous disease, Wiskott-Aldrich syndrome, IPEX, IPEX-like syndromes (CD25 deficiency and gain of function in STAT1 signaling), atypical SCID (severe combined immunodeficiency), OS, NEMO (nuclear factor kB essential modulator) deficiency, XIAP (X-linked inhibitor of apoptosis) deficiency, and autoinflammatory disorders (Tables 1 and 2) (4, 18, 19). This evidence concerns the gene XIAP and severe combined immunodeficiency.