Interleukin 1 receptor, beta subunit (IL1RB/IL-1RB) also known as Cytokine receptor family 2 member 4, Interleukin-1 receptor subunit 2, and cytokine receptor family II, member 4, is a subunit for the interleukin-1 receptor. IL1RB/IL-1RB belongs to the cytokine receptor family. It is an accessory chain essential for the active interleukin 1 receptor complex. Coexpression of this and IL1RA proteins has been shown to be required for IL1-induced signal transduction. Defects in IL1RB/IL-1RB are the cause of inflammatory bowel disease type 25 (IBD25). It is a chronic, relapsing inflammation of the gastrointestinal tract with a complex etiology. It is subdivided into Crohn disease and ulcerative colitis phenotypes. Crohn disease may affect any part of the gastrointestinal tract from the mouth to the anus, but most frequently it involves the terminal ileum and colon. Bowel inflammation is transmural and discontinuous; it may contain granulomas or be associated with intestinal or perianal fistulas. In contrast, in ulcerative colitis, the inflammation is continuous and limited to rectal and colonic mucosal layers; fistulas and granulomas are not observed. Both diseases include extraintestinal inflammation of the skin, eyes, or joints.