Monogenic defects of primary immunodeficiency can present as severe neonatal IBD, including epithelial barrier defects, phagocyte defects (Chronic granulomatous disease, defects of NADPH oxidase LAD), B and T cell abnormalities (Severe combined immunodeficiency, common variable immunodeficiency, Wiskott-Aldrich Syndrome, immunodysregulation polyendocrinopathy enteropathy X-linked syndrome), autoimmunity (including XLP2) and IL-10R defects. Here, FMO5 is linked to inborn error of immunity.