The Interleukin 1 receptor family (IL-1 R) comprises at least eleven members including IL-1 RI (IL-1 R1), IL-1 RII (IL-1 R2), IL-1 RAcP (IL‑1 R3), ST2 (T1/IL-1 R4), IL-18 Ra (IL-1 Rrp/IL-1 R5), IL-1 Rrp2 (IL-1 RL2/IL-1 R6), IL-18 Rb (AcPL/IL-1 R7), IL-1RAPL‑1 (TIGIRR‑2/IL‑1 R8), and TIGIRR-1 (IL-1 R9) (1). All family members possess three immunoglobulin (Ig)-like domains in their extracellular region. Most members have an intracellular TIR (Toll-like receptor/IL-1 receptor signaling) domain that is also conserved in the Toll-like receptor family. Five of the IL-1 R family members (1, 2, 4, 5, and 6) are clustered and localized to chromosome 2. SIGIRR (single Ig domain containing IL-1 receptor-related molecule) is a subtype of the IL-1 R family that differs from the other nine members by having only one Ig domain in its extracellular region. The sequence of mouse SIGIRR predicts a 409 amino acid (aa) residue transmembrane glycoprotein that lacks signal peptide and contains a 117 aa single Ig extracellular domain, a transmembrane region and a 268 aa cytoplasmic tail with a TIR domain. The cytoplasmic tail of SIGIRR contains a C-terminal extention beyond the TIR domain which is also found in IL-1 R8, IL-1 R9, and Toll-like receptor family members but absent in other IL-1 receptor family members. SIGIRR is widely expressed and is present in all cells and tissues examined. Mouse and human SIGIRR share 82% amino acid sequence identity. The ligand and signaling mechanism for SIGIRR has not been identified.