Interleukin 1 receptor accessory protein-like 2 (IL-1 RAPL2), also known as TIGIRR-1 and IL-1 R9, is a member of the IL-1 RI-like family of the IL-1 R/Toll-like receptor superfamily. IL-1 RAPL2 is a type I transmembrane protein that contains three Ig-like domains in its extracellular region and one intracellular TIR domain. IL-1 RAPL2 is expressed in the central nervous system, and its deletion is associated with some forms of X-linked mental retardation. An alternately spliced variant lacks the transmembrane and cytoplasmic regions. Within their extracellular domains, mouse IL-1 RAPL2 shares 99% amino acid sequence identity with human IL-1 RAPL2.