The Interleukin 1 receptor family (IL-1 R) comprises at least nine members including IL-1 R1, IL-1 R2 (IL-1 RII), IL-1 R3 (IL-1 RAcP), IL-1 R4 (ST2/T1), IL-1 R5 (IL-18 R alpha/IL-1 Rrp), IL-1 R6 (IL-1 Rrp2/IL-1 RL2), IL-1 R7 (AcPL/IL-18 Rb), IL-1 R8 (TIGIRR-2) and IL-1 R9 (TIGIRR-1). All family members possess three immunoglobulin (Ig)-like domains in their extracellular region. Most members also have an intracellular TIR (Toll-like receptor/IL-1 receptor) signaling domain that is also conserved in the Toll-like receptor family. A subtype of the IL-1 R family, SIGIRR (single Ig domain-containing IL-1 R-related molecule) which differs from the other nine members by having only one Ig domain, has also been identified. See IL-18 R alpha for IL-1 R5 and IL-18 R beta for IL-1 R7.